McIlroy got his title defence off to a brilliant start with three birdies in his first four holes to charge up the leaderboard.
But the world number three could not maintain that momentum through a wobbly back nine that left him four shots behind co-leaders Jim Furyk and Masters champion Adam Scott.
"I got off to a fast start obviously," McIlroy said. "I was three under through nine holes. I felt like I played really, really well on the front nine.
"Overall it was good, to shoot under par today was a solid way to get off to any major."
Struggling through a season that has had more downs than ups after switching his club brand to Nike in a lucrative deal reported to be worth $250 million, McIlroy arrived at Oak Hill sporting a new look, having chopped his mop-topped locks in favour of a more disciplined close-cropped coif.
After romping to victory by a record eight shots in last year's US PGA Championship at Kiawah Island, McIlroy appeared determined to once again run away from the field as he picked up a birdie at the first, and two more at the third and fourth.
The sprint, however, turned into a stumble with the 24-year-old collecting more bogeys (four) than birdies (two) over his final 14 holes.
"Made a couple of bogeys on the back nine and that sort of halted the momentum," said McIlroy, who claimed the order of merit on both sides of the Atlantic last year. "It's not like there are guys that are so far ahead.
"You go and get off to a good start tomorrow, you are right there."
With just four top-10s in 12 US PGA Tour starts this season, finding that type of consistency has been harder than settling on a new hairstyle for McIlroy.
"Consistency day-to-day or hole-to-hole, you focus on each and every shot and try not to think ahead too much or think about anything else," said McIlroy. "Really, just focus on my shot, that's what I'm trying to do."