PARIS • World No. 1 Rafa Nadal's participation in the season-ending Tour Finals hangs in the balance when the Spaniard pulled out of the Paris Masters with injury yesterday.

He told a news conference ahead of his quarter-final match against 77th-ranked Serb Filip Krajinovic that he was suffering from a right knee problem.

The 31-year-old said he would try his best to be present at the eight-man event which takes place from Nov 12-19 at the O2 in London, but his focus was on the long term.

"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 the 10-time French Open champion, who struggled with his knee in Thursday's third-round win against Pablo Cuevas.

He called the trainer to tape his right knee after losing the second set in his 6-3, 6-7 (5-7), 6-3 victory.

Nadal, who has struggled with knee injuries throughout his career, became the oldest man to secure top spot in the end-of-season rankings when he beat Chung Hyeon in his opening match on Wednesday.

"It's a tough situation but I tried my best," he added. "I did one treatment yesterday night to try and play today."

Nadal, who has never won the Paris Masters, did not know yet whether he would be fit for London.

"It's not about London, it's about the longer term," he explained. "It's important to play tennis as long as possible."

If Nadal pulls out of the Tour Finals, it would leave world No. 2 Roger Federer as the only member of the "Big Four" - which includes Novak Djokovic and Andy Murray - in the event.

Serbia's Djokovic has not played since Wimbledon in July after ending his season early due to an elbow injury and will drop out of the world's top 10 for the first time in more than a decade next week.

REUTERS, AGENCE FRANCE-PRESSE