As the weather worsened on Saturday, Henrik Stenson's lead diminished. Once ahead by nine in the third round, Stenson bogeyed three of the last five holes at The Tour Championship as he managed a 1-under 69 in round three.
Stenson, the Deutsche Bank winner, was nine clear as he made the turn at 14- under, but played the back nine in 3-over par en route to his 69.
The Swede ended 54 holes at 11-under-par 199.
Dustin Johnson, the last player to get into The Tour Championship field, carded a 3-under 67 to move into second place at minus-7.
Steve Stricker posted a 2-under 68 to climb into third at 5-under-par 205.
U.S. Open champion Justin Rose (70), Zach Johnson (69), Billy Horschel (70) and Jordan Spieth (71) share fourth place at minus-4 at East Lake.
Tee times were moved up to beat the weather, but the rain moved in as the leaders were late on their front nine, then got steadier and heavier as the round went on.
This is the finale of the FedExCup Playoffs and the winner of the Tour Championship has a chance to win both titles at the same time. Stenson is second to Tiger Woods on the points list and would win both crowns with a victory this week.
World No. 1 and two-time winner Woods managed a 1-under 69 to end 54 holes at 3-over-par 213. He is tied for 26th place.
Defending champion Brandt Snedeker carded a 3-over 67 and that helped him move into a tie for 22nd at plus-1. Since the FedExCup playoffs started, Snedeker is the first Tour Championship winner and FedExCup champ to qualify for the following year's Tour Championship.
Stenson kicked in a 3-footer for birdie at two and drained a 19-foot birdie putt at three to quickly move to minus-12. After three pars in a row, he birdied the seventh from seven feet out.
The three-time PGA Tour winner chipped his third at the par-5 ninth to two feet and knocked that in to climb to 14-under par.
Stenson, who is making his first Tour Championship start, was nine clear of the field at that point.
Johnson made an 18-footer for birdie on No. 4. He parred the other eight holes of the front side, then also parred 10 and 11.
Stenson found the rough off the tee at the 10th and that led to a bogey. Johnson made a 6-foot birdie effort at 12 to get within seven of the lead.
Johnson failed to get up and down for par from a bunker at the 14th, but Stenson also bogeyed that hole. Johnson bounced back with a 2-putt birdie on the par-5 15th, while Stenson chipped his third to three feet and he drained that birdie chance.
The 16th proved to be a two-stroke swing. Johnson dropped his approach five feet from the hole and rolled in that putt for birdie. Stenson badly missed the green from the left rough. He chipped to 21 feet and 2-putted for bogey to trim his lead to five.
Johnson got up and down for par on the final two holes. At the last, he saved par from over 30 feet out after his tee shot landed in a bunker.
Stenson scrambled for par at 17 as he sank a 15-footer to save par. On No. 18, his tee ball stopped 46 feet from the hole and he 3-putted for bogey to cut his lead back to four, which it was at the start of the day.
"I think I'll choose to look at it from the bright side, even though the weather is not that bright at the moment. Started the day with a 4-shot lead and I still got it," Stenson said. "So that's all that matters really."
Adam Scott, who needed an IV before his round, struggled to a 5-over 40 on the front nine. He picked up one birdie on the back nine for a 4-over 74, which left him tied for 10th at 2-under-par 208.
NOTES: Stenson failed to hold his two previous 36-hole leads, but this is the first time he has had the 54-hole lead on the PGA Tour ... Sixteen of the 26 third-round leaders at the Tour Championship have gone on to win, while only 16 of 37 third-round leaders have won this season on the PGA Tour.