Koepka - who started the day at 7-under with the new staggered-scoring format implemented for the FedEx Cup Playoff finale - carded a 3-under 67 on Thursday, which was capped by three birdies in his final four holes.
Thomas entered the tournament with a two-stroke lead at 10-under as the FedEx Cup points leader but failed to improve his score with an even-par 70. A costly double-bogey on the easy par-3 15th derailed Thomas' day.
Schauffele recorded the low round of the day, firing a 6-under 64 to quickly erase the six-shot deficit he started the tournament with. He won at East Lake in 2017 as a rookie.
The trio of leaders sits one shot up on Rory McIlroy, who improved his standing by four shots with a 66 on Thursday.
Here's how the top of the leaderboard looks after Round 1:
|Place||Player||To Par||R1 Score|
World No. 2 Dustin Johnson struggled to a 3-over 73. He fell into a tie for 23rd overall and is now 10 shots off the lead.
Friday's second round begins at 12:00 p.m. ET.