World No.1 Jason Day battled through back pain to be well placed in the chase for PGA Tour's richest payday after the opening round at the Tour Championship.
Day finished with a grinding three-under 67 at East Lake Golf Club to be just one shot from the lead and in a tie for fourth despite battling his back and a wonky driver.
His biggest recent rival, world No.2 Dustin Johnson, continued his hot form to also share the lead.
After withdrawing from his last start with an annular ligament tear in his back, Day continued to feel the pain, hitting just five of 14 fairways.
“I feel OK. There was a couple of drives out there where just off the top of the transition, felt a bit of a sharp pain in my back. That's why some of the drives went a little bit wide,” Day said.
“When I get to the top of the swing, as soon as my hips start to unfold and then there's that bit of separation, it just crunches down. It just hurts. I mean, it is what it is.”
While the threat of pain remains, Day’s competitive instincts are taking over as he looks to beat out Johnson and the rest of the field for the lucrative FedEx Cup.
The Queenslander can claim the season long title and $US10million ($A13 million) bonus that comes with it if he can win this 30-man shootout.
American Kevin Chappell and Japan’s Hideki Matsuyama joined Johnson in the lead with rounds of 66 to lead at four-under.
Korean Si Woo Kim and American Kevin Kisner joined Day in a tie for fourth one shot back.
“It's not an excuse,” Day said of his back pain.
“I've still got to play the golf that needs to be played to try and beat these guys. Unfortunately, they're not going to be sympathetic to me.
“The putter was nice today. It was on, which is good and to be able to shoot three under in some of the positions that I was in, get up and down where I was getting up and down from, I'm pretty pleased with.”
Adam Scott, who joins Day, Johnson, Patrick Reed and Paul Casey as the five players who are guaranteed the FedEx Cup if they win this event, was left to rue a late slide in his round.
At three-under through 15 holes, Scott posted back-to-back bogeys on 16 and 17.
And a short birdie miss on the 18th left him with a 69, tied 11th at one under.
“I can afford those mistakes hopefully on a Thursday, but no more hopefully,” Scott said.
“I played a good round of golf today. It's a hard course and under par is pretty good so I'm in good shape.”
Defending champion Jordan Spieth finished with a 68 as did Rory McIlroy and Casey to be tied seventh.
McIlroy rode a rollercoaster all day, carding seven birdies but also a bogey and two double bogeys.
Reed struggled to a 73.
