Dustin Johnson faces a race against time to be fit for next week's US PGA Championship.
The world No.1 has withdrawn from this week's AT&T Byron Nelson in Texas with a knee injury, casting a huge question mark over his participation in the second men's major of the season.
In a statement, 36-year-old Johnson said: "Unfortunately, the knee discomfort I occasionally experience has returned and, after consultation with my team and trainers, I feel it is best that I remain at home and focus on my rehabilitation work.
"I am not pleased about this situation, as I was really looking forward to playing this week. I have several friends in the Dallas area, many of whom I have met through their involvement with the tournament, so I hate to let them down. I am truly sorry I won’t be there with you."
The two-time major champion underwent surgery in 2019 to repair cartilage damage in his left knee, a procedure which sidelined him for several months.
The 24-time tour winner hasn't played since the Valspar Championship earlier this month, where he finished in a tie for 48th.
Johnson will be particularly keen to feature in the US PGA, with the tournament taking place over Kiawah Island's Ocean Course in his home state of South Carolina.
He has finished runner-up in each of the last two editions of the championship and has missed only three majors dating back to the start of the 2009 season: the 2012 Masters, the 2014 US PGA, and the 2017 Masters.
The 2021 US PGA gets underway on Thursday, May 20.