Swede Henrik Stenson held firm at rain-soaked East Lake Golf Club, while Jordan Spieth made a charge to stand three strokes back after Friday`s second round at the season-ending Tour Championship.
Stenson, who won the event wire-to-wire in 2013, made it six rounds in a row holding the lead at Bobby Jones` old stomping grounds near downtown Atlanta as he built on his two-shot overnight lead with a two-under 68 for a nine-under 131 total.
"So, a two-shot lead yesterday and a three-shot lead now after two rounds. Whenever you can maintain it or extend it is always a good thing," said Stenson, who fired a 63 in the opening round.
Masters and U.S. Open winner Spieth posted a bogey-free 66, the low round of the day, firmly rolling in a 20-foot birdie putt at the last to reach six-under-par 134.
Englishman Paul Casey bogeyed the last for a 70 to slip back to five under par, where he began the day in second place.
Spieth began to lock in the speed of the greens, needing only 23 putts as he rescued a number of pars with some brilliant up-and-downs with his vaunted short game.
"Around the greens, I feel comfortable on Bermuda (grass)," said 22-year-old Spieth, who missed nine greens but did not register a bogey. "This is exactly the stuff I grew up on. It`s as comfortable as I feel all year. I love the greens here."
Another stroke back was British Open champion Zach Johnson, who lost five shots to par in five holes from the ninth but recovered with three birdies in the last four holes for a level-par 70 that put him at four-under 136.
Four-times major winner Rory McIlroy (71) and Australian Steven Bowditch (69) stood six strokes behind Stenson at three-under, while world number one Jason Day shot 71 for level-par 140.
More than an inch and a half (4 cms) of rain fell overnight and light rain continued for most of the day, cutting down fairway roll, muddying balls, making the tricky rough harder to escape and playing havoc with reading the speed of the greens.
Sixteen players posted under-par rounds on Thursday, while only four managed to dip under 70 in the second round.
Double bogeys stalled the charges of several players including Casey at the par-three sixth, Johnson at the par-five ninth and Rickie Fowler (70 for 139) at the par-four 13th.
Even Stenson, who has ruled East Lake in his two appearances, stumbled.
After a day of pin-seeking approach shots in the opening round, the Swede left several approaches well short of the pins in the heavy air, and often rolled lag putts well past the hole as he made a pair of bogeys along with four birdies.
"I didn`t feel like it was my best day out there today, but I still managed to keep it together and two under par around here is never a bad thing," said 39-year-old Stenson.
"Conditions were tougher as well. So it was a slightly different golf course playing a lot longer than we normally see it, and I didn`t feel like I brought my best ball striking.
"But I still managed to keep it together, made a couple of nice putts out there and kept the momentum going."