Kei Nishikori reached the third round of the French Open on Wednesday after back-to-back five-set matches and over eight hours on the court in Paris.

The 31-year-old needed four hours to see off Russian No. 23 seed Karen Khachanov 4-6, 6-2, 2-6, 6-4, 6-4 on Court Philippe Chatrier.

He also needed four hours to beat Italian qualifier Alessandro Giannessi in the first round.

"I feel like I have played five matches already," Nishikori said. "Trust me, I hate playing five sets. I don't do it on purpose."

The statistics tell a different story.

Nishikori went to five sets in his only two matches in last year's tournament and is now 9-2 in five-setters at the French Open.

Overall, he has won 11 of his last 12 matches over five sets and has the highest winning percentage among active players in those matches.

"Today, my energy level was not great. I had a four hours first match, four hours today," he added. "After the third set loss, I kept telling myself I want to keep playing. There was no answer for me, but I just keep playing.

"My body said no, but my mind was just 'keep playing.' So I was able to win the fourth set without thinking anything."

Nishikori is 26-7 in five-set matches in his career.

The key now is to press on and turn his unquestioned stamina into a period of sustained success in a career that has so far yielded 12 titles.

"Everybody tells me I have a good record," Nishikori said. "Sometimes I have to think that I have to keep building my record after going five sets.

"I think in fifth sets I'm really playing good tennis, more than any other sets."

Nishikori has been ranked as high as No. 4 and was the U.S. Open runner-up in 2014. He's currently ranked No. 49, the lowest he has been since 2011.

However, it has taken some serious hitters to subdue him during this clay court season.

World No. 6 Alexander Zverev defeated him in Madrid and Rome. In between those losses, Nishikori took a set off Rafael Nadal before losing in three sets in Barcelona.

"I only lost to top-10 players. You can say I'm not beating top-10 players right now, but I think things are getting much better for me," said Nishikori.

"I think I'm on the way to the top 10 again. I'm really enjoying this moment."

Regardless of how far he gets at the French Open — he plays Swiss qualifier Henri Laaksonen next — Nishikori won't have a lot of time to dwell on his form.

His wife Mai is expecting their first child later this year.

"I'm still thinking about the name of my child. It will be a Japanese name," he said.

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