The trio is at seven-under 133 after the second round of the fourth and final tournament of the PGA Tour’s 2017 playoffs. Simpson and Casey each shot three-under 67 on Friday after starting their rounds tied for second place.
Thomas, who has won five times this season, hit his 231-yard approach shot on the 590-yard final hole to within six feet of the cup and made the putt for eagle.
“I really felt like I played or had the opportunity to really go decently low today or post a pretty solid number out there,” the 24-year-old Thomas said. “I had some good putts that I don’t know how they didn’t go in.”
Tied for fourth at six-under 134 is Patrick Reed, England’s Justin Rose (66), Gary Woodland (67) and Spain’s Jon Rahm (67).
Rookie Xander Schauffele (66), US Open champion Brooks Koepka (69) and Jason Dufner (67) are tied for eighth at five-under 133.
American Thomas can win the $10 million FedEx Cup playoff prize if he wins the Tour Championship. He finished sixth in the tournament last year.
“It’s nice to have everything in my own hands and I know that if I just take care of what I need to do, then it’s over with,” Thomas said.
Casey stumbled with back-to-back bogeys at 12 and 13, but then gathered himself and made a birdie at No. 16. He parred the final two holes.
“Dropping a couple of shots on the holes I did, it just shows you don’t need to do a lot wrong around here to make bogeys,” Casey said. “It would have been very easy to just kind of slip back in the middle of the pack and that’s not what I wanted to do.”
Former US Open champion Simpson had to overcome a double-bogey at No. 13 after a poor drive. Simpson made par on the his last five holes.
Top-seeded Justin Spieth is tied for 15th after following his first-round 67 with a 70. His first hole bogey was the first of four on the day.
World number one Dustin Johnson carded a 69, and is tied with Spieth at three-under 137, four shots adrift of the leaders.
“I felt like I hit it good, had a lot of opportunities, didn’t make any putts,” Johnson said. “I just want to get it within striking distance. If I can get it within three, I would be happy.”
First-round leader Kyle Stanley shot a three-over 73 and is also at three under at the halfway stage.