World No 1 Rafael Nadal pulled out of the Paris Masters on Friday before his quarter-final with a knee injury.
The Spaniard, who had his right knee strapped during his third-round win on Thursday, said he wanted to try and be ready to face Filip Krajinovic in the last eight but that "it wasn't possible to play".
He did not say if he would be fit to play at the World Tour Finals in London later this month, though in a news conference he did not sound confident.
"The way that I am today I do not see myself playing three more matches,” he said of the problem.
“The knee is always bothering me a little bit but sometimes it’s worse. For me it’s not about London, it’s about the longer term.
"I am going to do my treatment and my best to be playing in London."
Nadal is already guaranteed to finish the year as world No 1 after Roger Federer, the nearest man to him in the rankings, chose to sit out the Paris Masters.
But, if Nadal does not play in London, and Federer wins the tournament, winning all five matches along the way, then the Swiss player will start 2018 only 140 points behind his great rival.
Nadal's withdrawal means his record of having never won the Paris Masters goes on.
Krajinovic, a Serbian qualifier will now play in Saturday's semi-final, going up against the winner of Juan Martin del Potro and John Isner's quarter-final.