Rafael Nadal has withdrawn from the latter stages of the Paris Masters and may now miss the season-ending ATP World Tour Finals in London due to injury.
The 16-time Grand Slam winner was given a bye through to the second round in the French capital and later defeated Hyeon Chung in straight sets to ensure he will finish the year ahead of long-time rival Roger Federer as the men's world number one.
Nadal was hoping to triumph in Paris to move one clear of Novak Djokovic and claim his 31st ATP World Tour Masters 1000 title.
He battled to a three-set victory over Uruguay's Pablo Cuevas in the the round of 16 on Thursday (2 November), but appeared to suffer a recurrence of the persistent knee problems that forced him to withdraw from the Swiss Indoors in Basel earlier this month.
The 31-year-old, who had his right knee strapped against Cuevas, was due to take on Serbian Filip Krajinovic in the quarter-finals on Friday, but instead called a press conference to confirm that he would not be able to continue in the tournament.
"It's a very hard decision, a tough and sad situation," Nadal was quoted as saying by The Telegraph. "I tried my best. I had treatment yesterday night but it's impossible to go on court. I was here to win, I'm sorry for the fans and organisation, that is the same of Roland Garros, like family to me."
Nadal will now be considered a major doubt to take part in the ATP Finals, which are at London's O2 Arena between 12-19 November. The likes of Djokovic, Andy Murray, Stan Wawrinka, Milos Raonic and Kei Nishikori are all already sidelined, while Federer pulled out of Paris to rest after winning his hometown event.
"I'm not thinking about London," Nadal added. "I'm thinking long term, to be able to continue playing tennis."
Nadal was previously beset by knee and wrist injuries before a 2017 renaissance that has seen him win six titles and two more Grand Slams, at the French and US Opens, while also reaching the final in Australia.
When announcing his withdrawal from the event in Basel on medical advice, the Mallorcan said he was suffering from an "overstressing of the knee" that was present during a Shanghai Masters final defeat to Federer that ended his run of 16 consecutive victories.