The former world number one had consecutive double-bogeys midway through his round, preceded by three straight early birdies and followed by an even better late run of four successive birdies.
"When I get it going, I can really run with it and then when it goes the other way I struggle to get out of it a little bit but that's the nature of the way my game is right now, and how it has been for most of the year," McIlroy told reporters after carding a two-under 68 that left him two strokes off the lead.
"I'm seeing more good, which is great, and holing some putts, making birdies ... as long as I continue to do that, I should be okay."
McIlroy, who blew away the field with a brilliant, trademark closing 65 to win the Deutsche Bank Championship three weeks ago, observed that his ability to make plenty of birdies should stand him in good stead for next week.
"This sort of golf next week would actually be okay at the Ryder Cup," said the four-times major champion. "Make seven birdies and you're going to do okay."
On the American Ryder Cup front, an intriguing sub plot at this week's Tour Championship will be the battle to earn the final wild card spot on the U.S. team.
Eleven players at East Lake are trying to impress captain Davis Love, who will select his 12th man after Sunday's final round.
Kevin Chappell, the joint leader on 66, and Kevin Kisner (67) made promising starts, but twice Masters champion Bubba Watson has his work cut out after a mediocre 72.
World number seven Watson is the highest-ranked player not already on the U.S. team, and he arrived at East Lake knowing he needed a strong showing after being overlooked last week when Love made his first three wild card picks.
Despite his slow start, Watson remained bullish.
"I thought I played great," the American left-hander said. "My ball-striking was where I wanted.
"Around this course it takes one good round. It's a long marathon."
(Reporting by Andrew Both in Cary, North Carolina; Editing by Mark Lamport-Stokes)
