Paris: Japan's Kei Nishikori who swept into the second round of French Open on Sunday, admitted that he had been close to pulling out after struggling to shake off a kidney infection.

The 21-year-old, who saw off Taiwan's Lu Yen-Hsun 6-1, 6-3, 6-4 in his Roland Garros opener, said he only decided late last week to come to Paris.

"I was close to not playing. My stomach had been hurting, but I started to feel better so I decided to come and practise here.

"The kidney infection is now fine. I am taking medicine for it and it's getting better. Physically and tennis-wise, it's pretty good and I'm confident."

Nishikori suffered a nine-month lay-off in 2009 after undergoing surgery on his right elbow, but believes his setbacks are making him a stronger player as he looks to rise from his ranking of 60 to the top 40 by the end of the year.

"It was a tough time when that happened. It was my first time without tennis for a couple of months and I learned that tennis is important for me," he said.

"I know that I have to take more care of my body than before. I think my body is getting stronger and I am hitting the ball more powerfully."

Nishikori is also getting used to the demands of clay, having made the final on Houston's green clay in April as well as the last 16 on Barcelona's red clay before his kidney problems pegged back his progress.

He also believes that hiring Brad Gilbert - the former coach of Andre Agassi, Andy Roddick and Andy Murray - is also making him a more complete player.

"He's teaching me a lot about clay, to use more lobs and slices," he said.

"This year I have played well on the red clay on Europe. I have faced more claycourt players and am getting more confidence."

US-based Nishikori added that the problems in Japan, still reeling from March's devastating tsunami and earthquake, were also uppermost in his mind.

"I couldn't believe it when it happened. It was a very sad situation, but we are a strong nation and getting to be better. The people will fight even though they are stil having a tough time."

Nishikori was happy to have come through against Lu, the world number 56, who was a shock Wimbledon quarter-finalist in 2010.

"I started well but he played better in the second set and by the third he was hitting well, side-to-side and was more aggressive. But, overall, I played a pretty solid match."

Nishikori will next tackle Ukrainian 31st seed Sergei Stakhovsky, who defeated French qualifier David Guez 6-7 (7/9), 6-3, 6-3, 6-3, for a place in the last 32.