Rafa Nadal pulled out of the Paris Masters on Friday with a recurrence of a knee problem and could not confirm whether or not he will be fit in time to contest the year-ending ATP Finals in London later this month.
The world number one told a press conference ahead of his quarter-final match against Serbian Filip Krajinovic that he was suffering from discomfort in his right knee.
Nadal added that he was going to try his best to be fit for the Tour finals, which take place from November 12-19 in the absence of several injured top players including Andy Murray, Novak Djokovic, Stan Wawrinka and Kei Nishikori.
Nadal, who has suffered from knee and wrist injuries throughout his career, decided to withdraw on Friday morning after having his right knee strapped during his third-round win over Pablo Cuevas.
Nadal: "It's not about London, it's about the long term"
He did not say if he would be fit for the Tour Finals in London later this month.
"For me it's not about London. For me it's about longer term," the 31-year-old, whose withdrawal allowed Serbian qualifier Filip Krajinovic to reach the semi-finals, told a press conference.
"Yesterday (Thursday), the pain was very strong but it was not the moment to stop.
"I had treatment last night to try and be able to play today... But unfortunately it was impossible for me to return to the court in the same condition as last night. It is a sad day for me."
Nadal has fought back this year from a series of injuries, winning the French and US Open titles.
He became the oldest man to secure top spot in the end of season rankings when he beat Hyeon Chung in his opening match on Wednesday.
"I'm going to do my treatment, do my best to be playing in London, but I cannot talk about that now because (it) is a day that is tough enough for me to pull out from here," he added.
Tournament director Guy Forget was not so sure Nadal would be fit in less than two weeks' time.
"I believe he's quite sceptical about his chances of playing in London now, and he came to Paris all the same," Forget said.