Sky's the limit for record-breaking Kei Nishikori

Tokyo: You won't catch Kei Nishikori snarling, scowling at opponents or yelling at umpires, but the unassuming 24-year-old has a fire in his belly every bit as strong as the game's fiercest gladiators.

Having just become the first Japanese to crack the world's top 10 in men's tennis, the sky could be the limit for the record-breaking Nishikori -- if his fragile body holds up.

Where Rafael Nadal stares daggers across the net at rivals, Nishikori shuffles almost apologetically along the baseline, a toothy grin belying his swashbuckling style.

Nishikori, who has risen to nine in the world rankings, came agonisingly close to beating Nadal for the first time in seven meetings in last weekend's Madrid Open final before injury struck again.

In control at 6-2, 4-3, he felt a twinge in his hip and went on to lose the second set 6-4 before retiring at 0-3 in the third.

But after wins in Memphis and Barcelona already this year, the Florida-based Nishikori has demonstrated he has the weapons to topple the giants of tennis.

Shuzo Matsuoka, the first Japanese player to win an ATP title in 1992, believes Nishikori could even produce a shock Grand Slam triumph this year, which would give Asian tennis a further boost following the success of China's Li Na in the women's game.

"There's a new generation of players coming through to challenge the big four," he told AFP, referring to Nadal, Roger Federer, Novak Djokovic and Andy Murray.

"He has the ability. I've been watching Kei since he was 11. He has the touch of a genius, great imagination.

"He has shots in his locker you just can't teach," added Matsuoka. "Breaking into the top 10 is just the start."

'So tough':

Nishikori burst onto the scene as an 18-year-old by winning in Delray Beach as a 244th-ranked qualifier in 2008, and is a huge celebrity in Japan.

Despite winning five ATP titles to date, Nishikori's career has been blighted by injuries, most notably in 2009 when he needed elbow surgery and feared he might not play again. But Matsuoka tipped him to join the Grand Slam winners' club.

"You saw in the Nadal match, his creativity and speed," said Matsuoka, whose run to the 1995 Wimbledon quarter-final sparked a tennis boom in Japan.

"I don't think anyone doubts he can win Grand Slams."

Given his first racquet when he was five, Nishikori has come a long way since leaving his family home in mountainous Shimane prefecture, western Japan as a bashful 13-year-old and arriving at Nick Bollettieri's academy in Bradenton, Florida, unable to speak a word of English.

Earmarked for success at an early age by the Japan Tennis Association, Nishikori has repaid them in gold, proving a cash cow for the domestic game and this year propelling his country to the Davis Cup quarter-finals.

Meanwhile, the decision to hire former French Open winner Michael Chang as coach late last year looks an inspired one.

"Chang is exactly what Kei needed," said Matsuoka of the 1989 French Open champion. "It's not just about how much game you have, but also tactics and resilience."

Nishikori's best Grand Slam performance to date was the quarter-finals in Melbourne two years ago.

But if he continues on his upward curve, favourable draws await at the four majors, and Stanislas Wawrinka's Australian Open victory in January suggests the field is more open than previous years.

"Kei has the potential," said Japan's former Davis Cup captain Eiji Takeuchi, who also feels the high-flying Nishikori has a Grand Slam crown in him.

"He was always a shy kid but once he stepped on court, a switch flicked on inside him. He is so tough. He could be aiming for the top five already."