After starting the tournament with a two-shot advantage at 10-under-par, the top-seeded Scheffler stumbled with a one-over 71 as fellow American Collin Morikawa emerged as the big first-round mover.

Morikawa fired a career-best nine-under 61 to forge a tie for the lead with Keegan Bradley (63) and Norwegian Viktor Hovland (68) at 10-under in Atlanta.

The three-way lead was made possible by a late triple bogey from Scheffler, who is tied for fourth one-shot back of the leaders.

Day is equal 17th in the 30-man field at four-under after cutting the deficit from nine to six shots with a first-round 67.

Showing Day the way, Morikawa was joint-24th in FedExCup points standings with the Queenslander entering the season finale at East Lake Golf Club.

He had seven birdies and an eagle on the par-5 6th hole in a bogey-free round to charge up the leaderboard.

Morikawa birdied each of his final three holes, including an up-and-down from the greenside bunker at the par-5 18th hole, and finished one shot off Zach Johnson's course record of 60.

"I've heard both sides of we like (the format), we don't like it," Morikawa said of the handicapping system.

"But at the end of the day, if you play really good golf for four rounds and you win the tournament without the strokes, I mean, you can't be too mad about yourself, right?

"You played some good golf and you beat 29 other guys and that's kind of the goal this week. That was the goal at the beginning of the week, but obviously where I sit now, it's to keep pushing and not let my foot off the gas."

Scheffler took the lead as low as 13-under thanks to three birdies in his first six holes, but had three-putt bogeys at the 8th and 11th holes before stepping onto the tee at the par-3 15th.

After pulling his 5-iron left off the tee and into the water hazard, Scheffler had to take a drop and reached the green in three. He then three-putted again from inside 16 feet, lipping out a 4-footer to make a triple bogey.

"I guess it's a little bit of a blessing to have a pretty bad day and still be in the tournament," said Scheffler. "So, yeah, go out there and just keep fighting."

Morikawa's playing partner Adam Schenk made a stellar Tour Championship debut by shooting a bogey-free 63 to move up to eight-under and into a tie for fifth with Russell Henley (65).

Rory McIlroy reportedly injured his lower back earlier this week and played through the pain to shoot an even-par 70, winding up back where he started the tournament at seven under. He was tied with Jon Rahm of Spain (69) and Matt Fitzpatrick of England (67).