Dustin Johnson has tested positive for COVID-19.
The world no.1 has been forced to withdraw from this week's CJ Cup at Shadow Creek.
Johnson had exhibited symptoms, the PGA Tour say. He notified tour officials and a test was administered which returned positive.
His place in the field at Shadow Creek will go instead to the first alternate, world no.61 JT Poston.
The 2020 FedEx Cup champion has begun a period of isolation, with the PGA Tour saying they're offering him their full support.
"Obviously, I am very disappointed,” Johnson said. “I was really looking forward to competing this week, but will do everything I can to return as quickly as possible.
"I have already had a few calls with the Tour’s medical team and appreciate all the support and guidance they have given me.”
There'll be no Johnson this week, but Brooks Koepka is back.
The four-time Major-winner will end a two-month absence when he tees up on Thursday.
Koepka initially suffered a knee tendon problem last autumn, which then led to an issue with his hip. The 30-year old was eventually diagnosed with a torn labrum.
"I know I felt it at the PGA, or that’s when it became the worst, trying to manage it and play through it," Koepka said on Tuesday.
"Just one of those things where if you’re not getting any weeks off to actually treat it, it’s hard to treat it on the road and do everything you need to do. But we’ve got it handled now.”
He hasn't played since August's Wyndham Championship, but feels ready to take on Saddle Creek, saying, "I didn’t know how bad I felt until I actually feel good."