Shanghai - Juan Martin del Potro hobbled out of the Shanghai Masters last 16 on Thursday after hurting his right knee in an awkward fall.
The Argentine world number four, who had already been suffering with a virus, lost his footing at 5-5 in the first set against Croatia's Borna Coric.
The 30-year-old Del Potro had lengthy treatment and strapping put on his knee, but conceded the match after losing the first set 7-5.
It caps a miserable few days for the third seed, who lost Sunday's China Open final to unseeded Nikoloz Basilashvili and revealed that he had been under the weather during the upset.
Coric, the 13th seed, faces Australia's unseeded Matthew Ebden in the last eight on Friday.