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.

• Live scores, schedule and results
• Alerts: Get tennis news sent to your phone