Australian Jason Day has downplayed any concerns over his playing future after an on-going back injury prompted the world golf number one to pull out of the Tour Championship during the second round in Atlanta.
Day 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 early departure at East Lake.
"He withdrew as a precautionary measure. Jason should be fine with some rest after a long break in the offseason."
Meanwhile, Dustin Johnson said his score did not reflect the quality of his play, but it was good enough to lift him to a one-stroke lead over fellow American Kevin Chappell after the second round.
U.S. Open champion Johnson, the hottest player in the game after three victories since June, carded a three-under-par 67 in the season-ending event at East Lake.
He saved par from six feet at the final hole to post a seven-under 133 halfway total, while Chappell (68) missed a good birdie chance at the last to end the day on six-under.
Johnson and Chappell separated themselves from the pack, with third-placed Kevin Kisner and Japan's Hideki Matsuyama four strokes from the lead at three-under 137.
If Johnson wins on Sunday, he will also claim the FedExCup and the $10 million prize awarded to the winner of the season-long points race.
"I felt I played really, really solid today. I just didn't really get a lot out of it," Johnson told Golf Channel after a round that included five birdies and two bogeys on a course with punishing, wiry rough.
"I just didn't really get a lot out of (the round), some close looks I didn't make and I missed a short par putt on 17 and not birdying the last hole on 18, I felt I left a few shots out there.
"Three-under's a great score around here, I'm never going to be upset with that but I felt I played really well today."
The long-hitting world number two has elevated his game in recent months, winning his first major at the U.S. Open, followed by a World Golf Championships victory and a triumph at the BMW Championship in his last start.
He has harnessed his prodigious power with a stellar short game for what has often proved unbeatable.
"I've got a lot of control with my ball," he said. "This course is difficult. You've got to drive it straight, control your irons coming into the greens.
"I'm really comfortable with the swing. I'm swinging really well. The biggest key is just putting in the work."
The Tour Championship is the only event on the PGA Tour that hands out two trophies, one to the tournament winner and the other to the FedExCup champion.
Johnson started the week as top seed for the FedExCup and chances of snaring the trophy have been boosted by Day's withdrawal.
Despite the reassuring statement from Day's management, his injury is hardly good news for a player who has been plagued with injuries and illnesses throughout his career.
Before this week's tournament, Day said his back was an on-going 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.
"Unfortunately, I've got an annulus tear in my disc, annulus tear of the ligament. I've torn it once before, and I've retorn it again."
He also said after the opening round that he had felt pain on a few shots, but not constantly.
"There was a couple of drives out there where ... felt a bit of a sharp pain in my back," he said after the round.
Day's withdrawal eliminates him from contention to claim the 12 million dollar bonus awarded this week to the winner of the PGA Tour's season-long FedExCup points race.
Day's early departure at the Tour Championship came after he launched two full-throttled lashes with his driver at the par-four eighth.
He pulled his drive into the lake left of the fairway, took a penalty drop in the rough and then took another mighty swing with the driver and sprayed the ball into more rough.
He did not appear to flinch after either shot but after walking up to his ball right of the green, he decided to call it a day.
He shook hands with playing competitor Kim Si-woo and trudged back to the clubhouse.
There was no immediate word on whether he still planned to compete in two tournaments in his homeland in November - the Australian Open in Sydney and World Cup of golf in Melbourne.