London: Juan Martin del Potro defeated Kei Nishikori in straight sets at Wimbledon on Saturday, but warned the rest of the tour that the Japanese rising star was shaping up as a downright "dangerous" player.

The 2009 US Open champion beat the 19th seed 6-3, 7-6 (7/3), 6-1 on Court One to book a place in the fourth round.

But the Argentinian ninth seed said the result belied the 22-year-old's promise.

"Nishikori is a dangerous player on a grass court. He's very, very fast, so I tried to put the ball one more time in every point," Del Potro said.

"I played aggressive. I served really well. I didn't make too many mistakes with my forehand.

"So I feel confidence with my game. To beat Nishikori three sets in a row, it's important."

Del Potro has played and beaten Nishikori twice before, in 2008 and 2009 on the US hard courts.

"He's playing much better, for sure. Maybe he has a little bad luck in the tiebreak. If he won that set we would still be playing. But I was lucky.

"Sometimes you need a little luck to win these kind of matches, even if you play better.

"But Nishikori is playing much better than two or three years ago, and he's still playing better and better. So he could be very dangerous for the rest of the players."

Nishikori was Del Potro's second Japanese victim at Wimbledon, the 23-year-old having beaten Go Soeda in four sets in the second round.

It was Nishikori's first tournament since being forced to retire at Barcelona in April with an abdominal injury.

The highest-placed Japanese man since the rankings began in 1973 had lost in the first round on his three previous Wimbledon appearances but showed his true potential this time.

He was the first Japanese man through to the third round at the All England Club since Shuzo Matsuoka reached the quarter-finals in 1995.

However, his bid to become only the second Japanese man to reach the round of 16 at Wimbledon in the Open Era failed this time.

Nishikori was plagued by injury problems after impressing on his US Open debut in 2008. He missed most of the 2009 season with a right elbow injury.