Nishikori is in Hong Kong to promote ATP events in Asia. Five tournaments are held over three weeks; in Kuala Lumpur (where Nishikori won the men's singles title), Shenzhen, Tokyo, Beijing and Shanghai (where the Masters 1000 - the top tier of the world tour events - kicks off today). Tennis is fast gaining popularity in the region and there is no better ambassador for it than Asia's new hero.

The strolling circus boards the Aqua Luna junk, decked out in extra finery in honour of its celebrity passenger. As we set sail, the photographers put their cameras down, giving me the chance to have a proper chat with Nishikori. For my opening shot, I ask him where it all began.

"I grew up in Matsue, in Shimane prefecture, which is one of the smallest cities in Japan. Both my parents play tennis for fun and I have an older sister [Reina] and we started together. The first couple of times we didn't have a tennis court or a net, just hitting some balls. After that I would play with my parents, especially my dad, and then I went to the local tennis school."

With time, Nishikori's skill, and his love of the game, blossomed. The extent of his talent became apparent when, in 2001, he won the All-Japan Tennis Championships for Kids at the age of 11.

Two years later, Nishikori was spotted by talent scouts and invited to enrol at the prestigious IMG Academy, in Bradenton, Florida, in the United States. The academy - whose alumni include tennis greats Monica Seles, Jim Courier and Andre Agassi - has a reputation for being a star factory and for fostering an ultra-competitive environment.

Olivier van Lindonk, vice-president of tennis at IMG, which runs the academy as part of a global sports and media empire, is Nishikori's manager: "Kei was able to attend because of [former Sony chief executive] Masaaki Morita, who awarded him a scholarship. Morita wanted to launch a top male Japanese tennis player, so he started the Masaaki Morita Tennis Fund using his own money.

"His express aim," says van Lindonk, "was to find talented Japanese kids at a young age and send them abroad", because Morita believed that Japan was not a breeding ground for champions. "It wasn't a popular thing to say, in fact it was pretty controversial at the time, but now, obviously, he looks like a hero. Because he was right."

As van Lindonk explains, Japanese culture - with its conformity and rigidly hierarchical social structures - isn't conducive to individual success.

"Respect is a huge part of Japanese culture but that doesn't work on the court. It's a competition out there. To give an example, if you put a young player on the court with an old pro, and the young guy gets his ball in but the older guy says it's out, he won't challenge it. He'll just say, 'OK.' So with these kids, we have to break them down and then build them up again."

When Kei arrived in Florida, at the tender age of 14, he didn't speak a word of English. The students practised tennis in the morning, followed by three hours of school lessons - two hours of which were devoted to learning English - and more tennis in the afternoon.

"For the first couple of years it was really tough to communicate with my coaches and my friends because I couldn't speak the language," Nishikori says. "But I didn't feel too homesick because I was very happy just to play tennis all day.

"I think Japanese people like teamwork. Sometimes they don't try to be No1 or different to other people. That can be good but sometimes it means you don't get a dream or a strong goal. In the US, it's more like you have to stand out and you want to be a little bit different to other people. You have to be strong mentally as well."

So has life in the US influenced his personality as well as his tennis?

"Yes, I think so. I used to be a bit quieter and I think living in America has made me a little more open about talking to other people."

Emboldened by this admission, I ask him about his girlfriend. According to the rumour mill, Nishikori is in a serious relationship with Honami Tsuboi, who represented Japan in rhythmic gymnastics at the 2008 Beijing Olympics, and the pair may be contemplating marriage. Nishikori blanches at the question and says the press "make up so much stuff". He clearly doesn't want to talk about it so I return to the safe topic of tennis.

In 2007, Nishikori's funding stopped and he turned professional. His first ambition, nicknamed "Project 45", was to surpass the highest ranking achieved by Shuzo Matsuoka, who was Japan's most successful male player, having reached the global position of No46 in 1992.

In 2008, Nishikori clinched his first ATP title, at Florida's Delray Beach tournament, and broke into the top 100. That year he also beat the world No4, David Ferrer, to make it through to the last 16 of the US Open, and was voted ATP newcomer of the year.

In 2009, his progress stalled when he had to undergo surgery on his right elbow. He was out of action for 14 months.

"That was the first time I really learned his character," says van Lindonk. "Most athletes who know they're going to be gone that long don't keep training, they take five or six months off. But Kei was at the academy doing his rehab, doing his gym work, every day. The other thing is that he took care of his coach for the whole year, which most players would never do. I can tell you, from many years on the job, that there's not a lot of players who are really nice, good people. Kei, he's just a humble guy. He really appreciates what he has. It would be hard to find someone who doesn't like Kei Nishikori."

Back in the game in April 2010, Nishikori began working his way steadily up the rankings. Once Project 45 had been accomplished, in 2011, he set his sights higher: Project 10. He suffered some further setbacks with injuries but maintained an overall upward trajectory. In 2012, he reached the quarter-finals of the Australian Open and then became the first home-grown player to win the Japan Open.

Nishikori is known for being a quick, clever and tactical player.

"Today's tennis is all about big servers," says van Lindonk, "but that's not him. He's smaller than the average and he has to fight. The way I describe him is as a 'gentleman warrior'."

"Sometimes it's tough when I play with bigger guys like [Milos] Raonic and [John] Isner," says Nishikori, "but there are some things they can't do. I have more speed, better technique … everybody has different strengths. That's the fun thing about tennis."

Nishikori says he's now playing more aggressively and that he's stronger, both physically and mentally, than ever before. And he gives the credit to his new coach, Michael Chang, a Chinese-American player who became the youngest man to win a Grand Slam event, when he triumphed at the French Open in 1989, aged 17. Chang paired up with Nishikori late last year.

"It's great to have him as my coach," says Nishikori. "For sure he's one of the reasons I'm getting stronger. He's almost Asian so we kind of know each other and can communicate well. Michael has taught me to believe in myself, that I can beat top 10 players. I'm calmer on the court and my concentration is much better now."

We break off our chat because the Aqua Luna has arrived at Tsim Sha Tsui. ATP is hosting a round-table lunch at Hutong restaurant with a group of sports journalists. Afterwards, Nishikori gets whisked away to do television interviews so I grab the chance to sit down with van Lindonk.

Nishikori is one of the few players to earn more off the court than on it. According to magazine, which publishes annual earnings lists, he made US$11 million in the 12 months to June, significantly more than players of a similar rank. As his manager, van Lindonk is entrusted with negotiating sponsorship deals.

"[Nishikori is] allowing us to make him a lot of money," he says, with a smile. "In Japan, every major athlete has an affiliation deal. For the last four years, Kei's affiliate has been Nissin [the company that invented the pot noodle]. He's seen as part of the corporate family - he's their boy. The Japanese media always refer to him as Nishikori/Nissin, so they will write, 'Nishikori/Nissin beat Djokovic at the US Open.'"

Another sponsor is casual clothing firm Fast Retailing, which operates the Uniqlo brand. Nishikori's win against Djokovic saw a massive run on his signature Uniqlo polo shirt and the company rewarded him with a US$940,000 bonus.

Nishikori is mining a rich seam; Japan has the world's third-largest economy but there are relatively few home-grown, international superstars for local brands to pin their logos to. But while Japan offers easy pickings, van Lindonk says his goal has always been to establish international partnerships.

"That's the business plan - to put him on the map globally. So he has a deal with Tag Heuer, not with Seiko or Citizen. He endorses Jaguar, not Toyota. And he's sponsored by Delta Air Lines, not ANA. Of course, he's always going to be a Japanese tennis player, but we feel he can move the boundaries in that regard."

How, as Nishikori's manager, does van Lindonk strike a balance between generating sponsorship revenue and driving forward the player's tennis career?

"At the beginning of the year we set three blocks of five or six days in which we do all his business stuff. So it's contained. The sponsors get lots of advance warning so they can put it in their calendars. That's how we limit some of the craziness."

Managing media appearances is another key aspect of the job. What I am witnessing today is just the tip of a media iceberg.

Female Japanese tennis players have had some notable successes: Kimiko Date-Krumm reached a career high of world No4 in 1995. But as the first male star, Nishikori is garnering press attention, and public adulation, on an unprecedented scale in his home country. When he plays in Japan the tickets sell out within hours and his big wins are front-page news.

"There are about 20 media outlets that have a 'Kei Nishikori person' - a journalist who follows him around the world and their whole job is just to write about Kei," says van Lindonk. "It can cause difficulties because they tend to overanalyse everything. We try to make sure it doesn't influence him too much. It creates a lot of pressure for him, though - every time he achieves something they raise the bar, and ask him when he's going to make the next step up."

It's time to move on from Hutong so the entire media circus piles into a fleet of cars and heads to the next event - a press conference at the Upper House hotel, in Admiralty. Here I meet Alison Lee, the ATP executive vice-president who's responsible for the group's tournaments in Asia. She says Asian tournaments are growing in popularity every year, attracting more spectators and awarding 10 per cent of the annual ATP prize-money pool.

"We're at a very exciting point because from here on, it's only going to be up for men's tennis in Asia," she says.

"To have someone like Kei break through serves as an inspiration, not just to players in Japan but regionally. Now they can think, 'He's not six foot five; if he can do it, maybe I can, too.'"

Nishikori might change the game in other ways. Tennis has been dominated by the so-called Big Four (Roger Federer, Rafael Nadal, Djokovic and Andy Murray) for several years but a clutch of new contenders has started challenging the old guard. Nishikori's US Open match against Cilic was the first in 38 Grand Slam finals that involved none of the Big Four.

At the press conference, there is much talk of the Barclays ATP World Tour Finals (which take place in London next month). Only the top eight ranking players are invited to participate in the season-ending competition.

"Roger, Rafa and Novak have already booked their places and the competition for the remaining five places is tighter than we've ever seen it before," says Lee. "There are lots of young players coming up so we'll definitely see some new faces this year. If Kei qualifies, he'll be the first Asian man we've ever had at the finals."

"I'm really hoping to make it to the ATP final this year," Nishikori tells the room. "The Asia swing [at which the players can earn qualifying points] is really important."

His ultimate goal is, of course, to win a Grand Slam title.

"I feel a little closer than before."

Nishikori's next stop is the Ladies Recreation Club, where he rallies with local junior tennis hopefuls, is mobbed by excited fans of all ages, signs autographs and poses patiently for countless more photos. He then heads to his final engagement of the day, a cocktail party and meet-and-greet session, back at the Upper House.

I'm starting to wonder if all this PR might be more gruelling than four sets against Djokovic. Nishikori, gentleman warrior that he is, handles the attention with commendable grace. All eyes and camera lenses have been trained on him for the entire day and, understandably, he's starting to look tired.

He's still smiling though, and must surely rank as No1 when it comes to professionalism.