Douglas Robson

USA TODAY Sports

LONDON – By Japanese standards, Kei Nishikori is a renegade.

At 14, he left the protective womb of Japanese society and relocated to the IMG/Bollettieri Academy in Florida to pursue his tennis dreams.

But ingrained cultural mores can linger – and sometimes hinder.

As a rising professional, Nishikori sometimes found himself subject to the hierarchical mindset that frames Japanese relationships – where younger members of a group, or "kohai," are expected to treat older "senpai" members with deference and respect.

"After turning pro, I was feeling a lot of respect to everybody actually, especially top players," explained U.S. Open runner-up Nishikori this week at the ATP World Tour Finals.

On Friday he advanced to the knockout semifinal phase in his tournament debut. He faces No. 1 Novak Djokovic in Saturday's semifinals.

Fifth-ranked Nishikori added that the first time he faced Roger Federer, he couldn't fathom beating his idol.

"I wasn't go(ing) for (the) win actually," he said. "That was one of the problem(s) I had."

That problem, and the attitude that held him back, is no more.

In a breakout season, 24-year-old Nishikori became the first Asian-born man to reach a Grand Slam final, losing to Marin Cilic of Croatia in New York.

He has won a career high four titles and 54 matches (54-12) and will end the season no lower than No. 5, making him the highest ranked Asian man in ATP Tour history (since 1973).

And when it comes to crunch time, Nishikori is "senpai" to everyone.

Nishikori's winning clip of .793 (69-18) in deciding three-set or five-set matches is the best in the Open era, including a torrid, tour-high 21-2 this season.

No player with 50 or more career decisive-set wins – not Djokovic, Federer or Rafael Nadal, not Bjorn Borg, Pete Sampras or Jimmy Connors – is superior.

"His deciding set record is remarkable," wrote ESPN analyst Darren Cahill in an email. "Belief and fitness have been key to his rise this year."

The 5-10 Nishikori credited his clutch play to a combination of physical and psychological improvement, but he stressed the latter.

"I think it's more mental thing," he said. "I try to stay there all the time. Try to stay focus(ed) first couple games."

However deep-seated his inferiority complex, Nishikori said he drew inspiration from the retired two-time Grand Slam champion Li Na of China and Paradorn Srichaphan of Thailand, the first man from Asia to crack the top 10.

"I felt a lot of motivation from them," he said of the two pioneering Asian players.

Another bridge to success has been the influence of 1989 French Open champion Michael Chang, who Nishikori brought in to help coach him this year. That, and his improved fitness, have been crucial to his climb to elite status after back-to-back top-20 finishes the last two seasons.

While his body has betrayed him at times throughout his career, including 2014, he hasn't retired from a match or pulled out of a tournament since nearly upsetting Nadal on clay in the finals at Madrid in May.

At the U.S. Open, he survived back-to-back five setters and then dismantled Djokovic in a four-set semifinal before bowing out to Cilic.

The other half of his coaching tandem, Dante Bottini of Argentina, says that Nishikori needed time to mature, especially from a culture with popular proverbs like "The nail that sticks out gets hammered down."

"You have to respect everybody, not only the elders," says Bottini, who began working with Nishikori in 2011. "But when you are on the court, you don't respect anybody."

Bottini added that Nishikori used to start matches nervously but now when pushed to a deciding set relies on a Zen frame of mind.

"Sometimes I wish he could be a little more energetic but I know he is going into that deep relaxation," Bottini says. "I can see it in his body language."

Here in London, Nishikori has demonstrated the self-respect and explosive play that should make him a Grand Slam threat in 2015 and beyond.

In round-robin play, he lost to Federer but posted wins over last year's Wimbledon champion Andy Murray and perennial top-10 player David Ferrer, who substituted for the injured Milos Raonic.

"He has easy power and becomes more aggressive late in matches which makes him dangerous," wrote Cahill. "You can also see Kei shaking out his legs early in matches which is normally a sign of nerves and might mean he's a bit of a slow starter. Once he gets involved in the match he loosens up and lets his game open up."

In Djokovic, he faces the hottest player on tour, who clinched his third year atop the rankings by manhandling Tomas Berdych of the Czech Republic 6-2, 6-2 on Friday afternoon.

Djokovic, 27, is riding a 30-0 streak on indoor hard courts -- the third-longest indoors in the Open era behind John McEnroe and Ivan Lendl -- including a 6-2, 6-3 defeat of Nishikori in the semifinals at Paris this month. They are 2-2 in career meetings.

The other semifinal pits No. 2 Federer against Swiss compatriot Stan Wawrinka, the No. 3 seed, or eighth-seeded Cilic. They play later Friday.

Bottini says Nishikori has shed the cultural baggage he once carried.

"Everything comes with maturity," says Bottini. "He knows that he belongs."

The Top 5 (by percentage) best decisive set win-loss record (based on 50 career match wins) in the Open Era

1. Nishikori 69-18 79.3%

2. Borg 119-40 74.8 %

3. Djokovic 126-46 73.3%

4. Nadal 120-50 70.6%

5. Connors 229-100 69.6%

Full list

Source: ATPWorldTour.com