American Koepka had held a one-shot lead after a weather-delayed third round, during which six people were injured after two lightning strikes.
He's still one shot behind, and finally he has a leaderboard that makes sense to him.
Thomas - the BMW Championship victor - was even par through his first five holes and 12 under overall for the tournament, ahead of Rory McIlroy and Brooks Koepka. It's considered the largest prize pool every handed out at a single PGA Tour event. That's I guess what I was born to do.
"I got myself in the final group and never took the fight to Tiger. Just leave it all out there".
Sitting one shot back of Koepka at 12-under are Justin Thomas and Rory McIlroy.
No one else was closer than nine shots of the lead. They were treated and released later that night.
The 10-time tour champion will resume play with the rest of the field on Sunday for the remainder of round three and the start of the fourth round.
World No 1 Brooks Koepka sits on 13 under at the top of the leaderboard after the second round of the Tour Championship.
Because he started the tournament at 7-under par as the No. 3 seed, he was at 13 under going to the weekend.
Schauffele made the first professional hole-in-one of his career at the par-three ninth as part of a three-under 67, while McIlroy's 68 means the Northern Irishman is well in contention. Rory made his second consecutive birdie, holing out from over 20 feet, while Koepka lost his wayward drive and ended up with double bogey.
This year, under a new scoring format that had the players open the Tour Championship with staggered scoring, McIlroy left East Lake with a $15 million check.
"Xander has won the Tour Championship before, not the FedEx Cup".
"I really, really played well", said the victor of last week's BMW Championship.
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