The British Open champion reeled off four successive birdies from the 12th, where he hit his approach to five feet, then picked up another shot at the last for a five-under-par 66.
That left McIlroy, the pre-tournament favourite who won the 2012 PGA Championship by a record eight shots at Kiawah Island, just one stroke off the lead held by Americans Ryan Palmer and Kevin Chappell, and England's Lee Westwood.
"Walking on to the 12th tee I was muttering a few things to myself," McIlroy told reporters after racking up eight birdies in the first round.
"It was just great to hit that second shot the way I did, hit it in close. It's one of the toughest par-fours on the course and to make birdie on that hole made me feel better
"It sort of gave me a little kick-start for that little birdie stretch."
LEFT FUMING
The Northern Irish world number one had been left fuming after he ran up an ugly seven at the par-five 10th, where he hit his second shot out-of-bounds, only to follow that with a three-putt bogey at the par-three 11th.
"I was very, very hot. What happened on 10 really knocked me off track a little bit and it's one of only a few bad shots I've hit in weeks," said McIlroy, who is seeking his fourth major title.
"I just double-crossed my second shot and what I was really angry about was, okay, you make a seven, but you don't compound that error by making a bogey on the next hole.
"You have to take whatever you are feeling inside and try and turn it into a positive. I was 'hot' and it's (all about) trying to use that fire as a fuel to propel yourself forward."
McIlroy, who regained the world number one ranking from Australian Adam Scott with his victory at the WGC-Bridgestone Invitational on Sunday, was delighted with the maturity he displayed in overcoming his rocky ride over 10 and 11.
"I think it just shows where my game is mentally right now, that I was able to do that today," said the 25-year-old.
"Also one of the big things for me in major championships is to get off to good starts and if you look at my three wins in the majors I have always got off to a good start.
"It was important to do that today and hopefully I can continue to be up there for the rest of the tournament."
With his game in sparkling order, McIlroy birdied five of his last seven holes and knows it could have been a spectacular back nine had he not stumbled his way over 10 and 11.
"It could have been a very special nine, but I will take what I was given today, enjoy my dinner tonight and get back at it in the morning," he smiled.
"Where I am with my golf game right now, I'm hitting the ball great, seeing good shots out there and holing putts. It's easier to get back in the frame of mind after what happened on 10 and 11."
(Reporting by Mark Lamport-Stokes; Editing by Greg Stutchbury)
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