Jeju - Defending champion Brooks Koepka withdrew from the CJ Cup in South Korea before the third round on Saturday citing a knee injury.
"During the course of Friday's round, I slipped on wet concrete aggravating a previous injury to my left knee," said the world number one in a statement.
Koepka last month had stem cells injected into a partially torn tendon in his left patella, a knee injury that had been troubling him for months.
"After consulting with my doctor, I have been advised to withdraw from the CJ Cup and head home for further tests," he said.
"I appreciate everyone's concerns and support. I'll keep you posted as I learn more."
In Friday's second round, Koepka shot a three-over par 75 to blow his chances of repeating his win of last year at the US PGA Tour's $9.75 million tournament in South Korea.
Justin Thomas, the 2017 winner here, led on 13-under par after two rounds with Koepka, on even par, back in a tie for 51st place in the 78-player field.
In his pre-tournament press conference on Wednesday, Koepka had said there were no issues with his knee following the treatment in early September.
"My knee's fine, it feels good. It bugged me since March maybe, but everything's fine now," Koepka said.
"Did my rehab, my recovery and there shouldn't be any issues. There might be one more round of stem cells maybe before the end of the year during some off time.
"But other than that, I feel great. My knee feels good. It's the first time it's been feeling consistently good in a while."
The withdrawal of the major drawcard and 2018 winner before the weekend rounds at Nine Bridges Golf Club on the picturesque Jeju Island will come as a blow to the tournament.
Koepka is not entered for next week's inaugural Zozo Championship in Japan, but the injury setback will now cast doubts on his participation in the prestigious WGC-HSBC Champions in Shanghai the week after, where he was due to lock horns again with world number two Rory McIlroy.
Koepka caused a storm earlier this week when said he did not regard McIlroy as a rival, even though the Northern Irishman won the season-long FedEx Cup and the US PGA Tour Player of the year accolade.
"I've been out here for, what, five years. Rory hasn't won a major since I've been on the PGA Tour. So I just don't view it as a rivalry," Koepka, who like McIlroy has won four majors, told AFP.
McIlroy's last major victory came at the 2014 US PGA Championship, while Koepka was still playing on the European Tour.
"I'm not looking at anybody behind me," said Koepka.
"I'm number one in the world. I've got open road in front of me I'm not looking in the rearview mirror, so I don't see it as a rivalry."