Rafael Nadal pulled out of the Paris Masters on Friday because of a right knee injury, putting in doubt his participation in the ATP finals.
The top-ranked Spaniard, who had his knee taped during the third set of his third-round win over Pablo Cuevas on Thursday, said he wouldn't be able to play Serbian qualifier Filip Krajinovic.
"I have to pull out. Of course it's a very tough decision for me, especially here in Paris, which has been the most important city in my career without a doubt," said Nadal, a 10-time French Open champion. "It's a tough situation but I tried my best ... I did one treatment yesterday night to try and play today."
He didn't say whether he will be able to play at the ATP finals in London, which starts No. 12.
"It's not about London, it's about the longer term," Nadal said. "It's important to play tennis as long as possible"
The 31-year-old Nadal said he will consult a doctor to assess the state of his right knee.
"I'm going to go back, check again with the doctors [like] I did after Shanghai," Nadal said. "Let's see. I'm going to do my treatment. In the past it worked well."
As the match against Cuevas went on, Nadal's movement looked restricted and he lacked his usual speed and mobility around court. While he was serving for the match, Nadal hardly had any spring as he double-faulted.
"Yesterday, the pain during the match was too much. But I didn't want to stop," Nadal said. "It's not the thing that I love to do."
Nadal missed large chunks of last season with a wrist injury and came back to win the French Open for the 10th time and the U.S. Open for the third time, taking his tally to 16 Grand Slam titles.
He began the year with a five-set loss to Roger Federer in the Australian Open final. Federer has beaten Nadal three other times this year.
Nadal has also been troubled by knee injuries in the past and preferred not to take any risks by continuing to play at the Paris Masters, where he has never won. He has only reached the final once, on his debut in 2007, losing to David Nalbandian.
But Nadal had already done enough in the tournament to finish No. 1 at the end of the year for the fourth time in his career, and first since 2013.
Nadal secured the world No. 1-ranking for the fourth time in his career on Wednesday when he defeated Hyeon Chung 7-5, 6-3 in the second round.