Louisville - Thailand's Kiradech Aphibarnrat withdrew from the 96th PGA Championship on Friday due to a knee injury.
Kiradech, playing alongside Americans Scott Piercy and Cameron Tringale, played only eight holes of his second round before pulling out at rain-soaked Valhalla Golf Club.
The 25-year-old from Bangkok, ranked 116th in the world, won his only European Tour title at last year's Malaysian Open. He has missed the cut in four of his past five starts.
The Asian standout becomes the fifth player to leave the field of 156 prematurely this week, the rest being Americans.
David Toms and Matt Kuchar pulled out before the start with back problems. Jason Dufner, last year's winner, withdrew after 10 holes Thursday with a nagging neck injury, and Ben Crane cited a back injury for his departure Friday morning after opening on 74.
Former world number one Tiger Woods withdrew from the last round of a warm-up event last week in Ohio due to a back injury but said he had no back problems in firing a 74 Thursday.