Billy Horschel, seeking a second consecutive PGA Tour victory, birdied three of the last eight holes to move two strokes clear of the chasing pack in the Tour Championship second round.
The American, ranked second in the FedExCup standings coming into this week, fired a second successive four-under par 66 at East Lake Golf Club to seize control of the fourth and final playoff event with an eight-under total of 132.
However, Northern Irish world number one Rory McIlroy was in hot pursuit after carding a best-of-the-day 65 in hot and humid conditions to soar into a three-way tie for second at six under.
McIlroy has already enjoyed a brilliant 2014 campaign, clinching two major titles among four wins worldwide, and has now set his sights on claiming playoff honours on the US circuit for the first time after a season-long race.
"I just want to finish my season off the way I feel like I should and the way I feel like it deserves to be finished off," McIlroy told reporters after piling up six birdies and a sole bogey with a sizzling display in the second round.
"I'm not putting too much pressure on myself because it's already been a great year, but I still want to win this thing."
Level with McIlroy at six under were Australian Jason Day (67) and American Chris Kirk (68), the FedExCup points leader who had been joint pacesetter with Horschel after the opening round.