FedEx Cup leader Chris Kirk and Billy Horschel share a one-stroke lead after round one of the season-ending Tour Championship.
Kirk - winner of the Deutsche Bank Championship - and last week's BMW Championship victor Horschel both carded four-under-par 66s at the East Lake Golf Club on Thursday.
Both men produced flawless rounds, holing four birdies apiece throughout 18 holes.
The American pair are two of five players with destiny in their own hands in Atlanta, with a US$10million bonus on offer if they can win the season-ending event to top the points list.
Bubba Watson - third in the standings - is a shot adrift at the end of day one alongside Patrick Reed, Jason Day and Jim Furyk.
Watson had seven birdies in the first round, though he double-bogeyed the 13th hole and bogeyed at the 18th for a 67.
Fellow American Reed, who started the day ranked 18th, birdied his opening hole before adding a further three by the end of his round.
Day - 10th in the FedEx Cup standings - faced a race against time to be fit for the season finale after withdrawing from the BMW Championship due to a back injury last week but the Australian showcased his undoubted quality, with his five birdies outweighing his two bogeys.
American Furyk also endured a mixed round, though he birdied four of his final seven holes.
The American quartet of Cameron Tringale, Zach Johnson, Bill Haas and Matt Kuchar are two strokes off the pace in a tie for seventh.
World number one Rory McIlroy and Adam Scott are among the five men level at one under.
Northern Irishman McIlroy - projected to finish fourth in the FedEx Cup - had four birdies and three bogeys for a 69.
Scott's inconsistent form continued after faltering late in the day.
The Australian - ranked 12th before teeing off - birdied four consecutive holes starting at the 12th but back-to-back bogeys at the 17th and 18th saw him slip off the pace.
American Ryan Palmer and Spaniard Sergio Garcia are also one under through 18.
Meanwhile, Hunter Mahan faces an uphill task in his quest to win the FedEx Cup.
Mahan - fifth in the standings at the start of play - shot seven bogeys for a four-under-par 74 to be tied for 27th.