ATLANTA • Jason Day downplayed concerns over his playing future after an ongoing back injury prompted the world No. 1 to pull out of golf's Tour Championship during the second round on Friday.
The 28-year-old also withdrew during the final round of his previous tournament, the BMW Championship, two weeks ago.
"Jason has a strained ligament in his lower right back with muscle spasm," his management company said in a statement shortly after his early departure at East Lake Golf Club in Atlanta, Georgia.
"He withdrew as a precautionary measure. Jason should be fine with some rest after a long break in the off-season."
There was no immediate word on whether he still planned to compete in the two tournaments in his homeland in November - the Australian Open in Sydney and the World Cup in Melbourne.
Despite the reassuring statement, Day's injury is hardly good news for a player who has been plagued with injuries and illnesses throughout his career.
On Wednesday, he said his back was an ongoing problem that flared up occasionally. "I had a bulged disc in 2014," he told reporters.
"Through the exercise that I've been doing, that bulged disc... has actually come in, which is kind of a miracle because you're thinking that it's either going to stay the same or get worse."
Day's withdrawal eliminates him from contention to claim the US$10 million (S$13.6 million) bonus to be awarded to the winner of the PGA Tour's season-long FedExCup points race.
American Dustin Johnson edged closer to the prize after firing a three-under 67 to seize a one-stroke halfway lead.
US Open champion Johnson, who arrived at East Lake atop the play-off standings after his win at the BMW event two weeks ago, fired five birdies with two bogeys for a seven-under total of 133.
He was one shot in front of fellow American Kevin Chappell, who started the day in a three-way tie for the lead with Johnson and Hideki Matsuyama and signed for a second-round 68 to total 134.
It was three shots back to the Japanese golfer, who shot a one-over 71, and Kevin Kisner (70) on 137.
"I felt like I played really, really solid today and just didn't really get a lot out of it," Johnson said.
"Still, obviously three-under is a great score around here. I'm never going to be upset with that."
England's Paul Casey (70), Northern Ireland's Rory McIlroy (70) and American Ryan Moore (68) were tied for fifth on 138. Defending champion Jordan Spieth (72) was in a group on 140 that also included Australian Adam Scott (71).
AGENCE FRANCE-PRESSE, REUTERS
Day 4: Ch204, tomorrow, midnight