Rory McIlroy played through the pain to shoot an even-par 70 and sit three off leader Collin Morikawa after the first round of the Tour Championship in Atlanta.
McIlroy - who injured his lower back earlier this week - started the day on seven under and that's where he ended it.
He went out in two-over 37 before getting his first birdie of the day at the par-four 10th when his approach shot stopped two feet from the cup. However, he gave the stroke back with a bogey on the next hole.
The 34-year-old hit a groove late, birdieing 13, 15 and 16. But his third shot at 18 sprayed from the rough to a bunker, and he went on to finish with a bogey.
McIlroy is tied with Jon Rahm (69) and Matt Fitzpatrick (67).
He said he is "over the moon" to be placed where he is considering the spasms.
"I was at the bottom of a squat, a body-weight squat, and my whole lower back spasmed, seized up. I couldn't move. I honestly couldn’t address the ball this time yesterday," McIlroy said.
"So, yeah, I mean, I hung in there and I just felt like if I could get through today, it’s better than it was yesterday, hopefully tomorrow’s better than it was today, and just sort of try to keep progressing."
He added: "So I was always going to tee off. It was just a matter of how I felt on the course.
"And it got progressively a little tighter as I went, but it will hopefully get loosened up here and just another 20, or 18 hours of recovery and go again."
Morikawa overcame a nine-stroke handicap to shoot to the top of the leaderboard.
As one of the lowest-ranked qualifiers in the 30-player field, Morikawa started the tournament on one-under-par but his round of 61 gave him a share of the lead.
Keegan Bradley, who started on three under, shot 63 and Viktor Hovlund recorded 68 from eight under to secure a three-way tie at the top.
World number one Scottie Scheffler, who started with the biggest handicap of 10 under, double-bogeyed the par-three 15th in a one-over round of 71, but he remains in contention one stroke off the lead.
Tyrell Hatton, one of five players to start on evens, worked his way up to a tie for 10th place with a six-under 64, but Tommy Fleetwood laboured to a one-over 71 to finish tied for 22nd place on two under.
