Defending champion Brooks Koepka withdrew mid-tournament from the CJ Cup ahead of his third round with a knee injury.
Koepka underwent stem cell on his left knee during the off-season to fix a partially torn patella tendon, which he said ahead of his missed cut at the Shriners Hospitals for Children Open had been affecting him since March.
The world No 1 told reporters on Tuesday he believed he was '100 per cent' ahead of his title defence and opened with a three-under 69, only to see his injury flare up during a second-round 75 and decide to pull out of the event.
In a statement, Koepka said: "During the course of Friday's round, I slipped on wet concrete, aggravating a previous injury to my left knee.
"After consulting with my doctor, I have been advised to withdraw from the CJ Cup and head home for further tests. I appreciate everyone's concerns and support. I'll keep you posted as I learn more."
The PGA Tour made the announcement ahead of Koepka's 08.15am (11.15pm GMT) tee time in South Korea, where he was 13 strokes behind halfway leader Justin Thomas and due to go out alongside Sergio Garcia and KJ Choi.
There was no immediate confirmation of the seriousness of the injury, how long Koepka could be ruled out for and whether he will feature at the WGC-HSBC Champions in China, which begins on October 28.