TOKYO: Former tennis great Andre Agassi on Saturday backed Japanese superstar Kei Nishikori to become a grand slam winner, hailing him as one of the greatest shot-makers ever.

Nishikori has climbed to number five in the world after a breakout year in which he won four titles and reached US Open final, but Agassi insisted it was only a matter of time before the swashbuckling 24-year-old captures one of the four majors.

“Listen, he was one match away from US Open,” the former world number one told AFP after going toe-to-toe with Nishikori in an exhibition event in Tokyo.

“Probably the biggest reason for (not winning the title) was the physical wear and tear over the course of the tournament, and sometimes that's a bit unlucky. “

“There is no question in my mind he believes he can win a slam,” added Agassi, 44, who won eight grand slam titles in an illustrious career before retiring in 2006 -- the year before Nishikori made his professional debut.

“I've seen him play with that purpose. From a talent perspective, he's very capable of winning a grand slam. He's dynamic and incredibly fast.”

Agassi, who smacked several eye-popping shots past Nishikori before going down 8-6 in an abbreviated match, insisted that the Japanese had the weapons to topple the game's big three of Novak Djokovic, Roger Federer and Rafael Nadal.

“He beat Djokovic on the way to the US Open final, and if he did it once he can do it again,” said the flamboyant American, tipping Nishikori to be at the forefront of any changing of the guard in men's tennis.

“There is always a time for seasons to change and he's positioned to be the one to step in. He's one of the few players I would pay to go and watch, he's that good.

'Greatest shot-maker'

“What he's done is very impressive in this day and age to do the way he does it. Kei is one of the greatest shot-makers the game has ever seen and there's no reason he can't go on further.”

Agassi, famous for his punishing training routine, warned Nishikori about the dangers of burn-out, however, offering some sage advice on physical preservation to a player who was plagued with injuries earlier in his career, most notably in 2009 when he feared he might not play again after undergoing elbow surgery.

“I know how hard you have to work to make it look easy at times,” said Agassi.

“Being number one and doing it over the course of a career requires good decision-making, not just talent. He's somebody who has to be mindful of his body, so that he's healthy all year.

“In tennis you don't have to play great -- you just have to play better than one person. Kei can play great when he doesn't have to, which is a good thing. But mentally it's much easier when you just play good enough against most of guys you can beat. It also helps your durability -- mentally, emotionally and physically.”