Brooks Koepka pushed into a one-stroke lead at the halfway mark of the Tour Championship on Friday.
The world number one carded a three-under 67 during the second round at East Lake, getting to 13 under and into the lead ahead of Justin Thomas (68) and Rory McIlroy (67).
Scoring was very difficult as only five players managed rounds of 67 or better, highlighted by Chez Reavie's six-under 64.
Koepka finished his round with a birdie at 18 to move one shot clear, having picked up shots from holes six through eight before bogeying the 13th.
Thomas, who was first in the standings to begin the FedEx Cup tournament, has struggled to find a rhythm through the first two rounds and has seen his two-shot lead disappear through 36 holes.
His bogey at the 17th dropped him out of the lead on his own but he was able to go into the clubhouse tied with McIlroy and Koepka.
However, the world number one's birdie at 18 was enough to get him out in front for the tournament.
McIlroy followed up his opening-round 66 with a three-under 67, which included a birdie despite ending up in the trees off the tee at 18.
Xander Schauffele (69) is fourth at 11 under, two shots ahead of Paul Casey (67) and four clear of Patrick Cantlay (71) as the top five threaten to pull clear.
Reavie is at six under alongside Patrick Reed (70) and Matt Kuchar (72) and produced a brilliant ace on the par-three ninth.