World number four Juan Martin del Potro could miss the rest of the season after fracturing his right kneecap in a heavy fall at the Shanghai Masters.
The 30-year-old Argentine retired from his last-16 match against eventual finalist Borna Coric on Thursday after falling during the first set.
Former US Open champion Del Potro, who has his leg in a splint, said: “It’s a very difficult moment. I feel very sad.
“It’s a hard blow that leaves me without strength.”
Del Potro had been expected to play in the season-ending ATP World Tour Finals in London in November.
“It’s very difficult for me to think about recovery again, I did not expect this to happen,” he said.
Del Potro has been plagued by injuries since winning his only Grand Slam in 2009.
Serb Novak Djokovic defeated Croat Coric 6-3 6-4 in Sunday’s final in Shanghai.