Dustin Johnson looked set to pull clear of the field at the Tour Championship, but his inconsistent finish means there is all to play for at East Lake.
A late wobble from Dustin Johnson handed Kevin Chappell a share of the lead at the Tour Championship ahead of Sunday's final round and left the battle for the tournament and the FedEx Cup wide open.
U.S. Open champion Johnson twice reached 10 under on Saturday and looked set to take a healthy advantage into the last 18 holes when he responded to back-to-back bogeys by birdieing the 15th to reclaim a two-stroke advantage over Chappell.
However, a three-shot swing followed on 17 - Chappell making birdie as his playing partner ran up a double bogey.
Johnson responded strongly by getting up and down from a greenside bunker to make birdie at the last and complete a 69, ensuring he and Chappell (68) ended the day on level terms at eight under.
The leaders are two clear of Ryan Moore and Rory McIlroy, who both shot 66. Both men have plenty to play for this week, with Moore hoping to earn a Ryder Cup berth and McIlroy still in the hunt for the FedEx Cup title and the $10million bonus that comes with it.
When Johnson reeled off three birdies in succession from the fourth, there looked to be no stopping the in-form American.
However, a bogey at seven followed and he then experienced mixed fortunes on the back nine to open the door for his rivals.
Chappell has yet to win on the PGA Tour, but held his nerve impressively on Saturday in a bogey-free round, while McIlroy - the winner of the Deutsche Bank Championship earlier this month - was at his brilliant best when it came to ball-striking.
The world number three knows victory this week will not necessarily be enough to secure FedEx Cup glory, should Johnson finish outright second.
"I shot a good one today and that's all I could do," McIlroy told NBC after his round. "I think first and foremost I need to go out tomorrow [Sunday] and just try to win the golf tournament."
McIlroy's third-round score was matched by Moore, who birdied four of the first seven, and Hideki Matsuyama stayed firmly in contention at five under, despite making a triple-bogey seven on the 14th in his 68. That slip-up was followed by birdies for the Japanese on the 15th, 17th and 18th.
Jason Dufner (66) sits five off the pace in sixth, one clear of Paul Casey (69) and Charl Schwartzel (66). The South African was significantly boosted by successive eagles at the sixth and seventh.