Juan Martin del Potro suffered a fractured kneecap at the Shanghai Masters in an injury which could rule the Argentine out of next month's Nitto ATP Finals in London.
The world No 4 retired from his third-round match with Borna Coric on Thursday and medical tests have shown he sustained a fracture of his right patella bone.
Del Potro said on atpworldtour.com: "It's a very difficult moment. I feel very sad. It's a hard blow that leaves me without strength. It's very difficult for me to think about recovery again, I did not expect this to happen."
The 30-year-old has reached the final of the US Open and China Open in recent months.
After quitting the match with Coric last week, he described the moment he hurt his knee.
Del Potro said: "During the match I had a bad fall. I made a weird movement with the knee and hit the floor very hard. I felt intense pain."
It is not yet known how long Del Potro will be sidelined for, with an ATP statement saying doctors will "evaluate the recovery process in the coming days."
Next month's season-ending ATP Finals, live on Sky Sports, take place from November 11-18, while the first major of 2019, the Australian Open, begins on January 14.
Del Potro is currently third on points in the Race to London.
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We will have coverage of the ATP Finals at London's O2 in November with every match live on Sky Sports.