The 14-time major victor, bidding for his first PGA Tour win in five years at East Lake, sunk a 27-footer to join Rickie Fowler at the top of the leaderboard.
Jason Day admits he is hungry to return to golf's spotlight after taking a back seat to a vintage Tiger Woods on day one at the Tour Championship.
Woods sits atop the leaderboard in Atlanta at five-under alongside Rickie Fowler (65), while Gary Woodland (66) is third at four-under.
Sixteen players in the 30-man field broke par on a hot afternoon with only a light breeze, and Thursday was all about staying in the game, especially as it relates to the chase for the FedEx Cup. The only way he can win the overall title - he won it in 2007 and 2009 and is the only player to do so twice - is to first win the Tour Championship and then have various outcomes among players near the top. Justin Thomas (3 under) and Dustin Johnson (tied for 12th, 1 under) are the other two.
"I'd never had an injury before", said Thomas, whose wrist was wrapped in medical tape at his pre-tournament news conference. "But to have come off the last few years of little activity and to have qualified for East Lake and to have been as consistent as I've been".
Tropical Storm Kirk speeds up as it moves west across Atlantic
Meanwhile, Tropical Storm Kirk chugged across the southern Atlantic on a westward heading toward the Lesser Antilles. Kirk is packing 40 miles per hour sustained winds and its westward motion is expected to accelerate through Tuesday.
McIlroy had held the early lead after carding four birdies in the first six holes, but eventually had to settle for a three-under-par 67. Woods did not win the last tournament in which he grabbed the first-round lead (finishing T-7 at the BMW).
There is a lot of golf left to be played - Kyle Stanley led after Round 1 here a year ago following a 64 and went on to finish T20 - but Woods and Fowler are in an enviable position. Houses along the side streets outside East Lake, and even one funeral home, offered parking and their yards were packed. The current world No. 8 will next week also celebrate making his Ryder Cup debut and DeChambeau could be partnering Woods in Versailles.
The Olympic champion hit 17 of 18 greens in regulation on Thursday and made three birdies and one bogey on both the front and back nine.
"The shots I missed were on the greens, all had attractive angles".