Dustin Johnson trebled his one-shot overnight lead at the Tour Championship but failed to hold on as a slow finish saw his advantage slashed by Kevin Chappell.
The US Open champion was looking good value to secure victory and remain on track for the £7.7m prize money on offer in the FedEx Cup, only for dropped shots at the 14th and 15th to cost him.
There was worse to come for Johnson on the penultimate hole, too, with a wayward tee contributing to two more dropped shots to open the door for the chasing pack in Atlanta.
Chappell was among those to take advantage, carding two birdies and not dropping a single shot during Saturday's session to draw level with his compatriot at the summit.
Rory McIlroy, another in contention for the large prize fund, is two shots further back in a tie for third thanks to a bogey-free 66, while Ryan Moore is also in with a shout of pipping Johnson.
