Dustin Johnson moved a stroke clear at the halfway stage of the Tour Championship, where defending champion Rory McIlroy lost ground.
FedEx Cup leader and world number one Johnson carded an even-par 70 to claim the outright lead at 13 under through 36 holes in Atlanta on Saturday.
Johnson was tied for the lead alongside world number two and BMW Championship winner Jon Rahm heading into the second round at East Lake.
But Northern Trust champion Johnson stood alone atop the leaderboard in the FedEx Cup play-off finale following four birdies and four bogies on a mixed day as Rahm could only shoot a 74.
Johnson – a runner-up in two of his last three starts – went 13 holes without hitting from the fairway, but the American star still managed to earn a slim lead, ahead of South Korea's Im Sung-jae.
Im, who is looking to become the second youngest winner of the FedEx Cup, birdied three of his last four holes for a six-under-par 64.
Last year's Tour Championship runner-up and 2017 winner Xander Schauffele is two shots adrift after reeling off three consecutive birdies late in the round.
Justin Thomas (71) – the 2017 FedEx Cup champion – is a stroke further back, while Collin Morikawa (65), Tyrrell Hatton (66) and Rahm are nine under heading into the third round.
After producing the equal best round of the opening day, four-time major champion McIlroy was unable to replicate his exploits following a one-over-par 71.
FedEx Cup and Tour Championship titleholder McIlroy, who recently became a father, struggled as the former world number one bogeyed three holes, including the last.
McIlroy's woes were highlighted by a shot that found water, when he tried to reach the 18th green from the thick rough but the ball went into the bottom of the lake.