Unlike last year, when he romped to victory with a record-breaking total of 16-under-par at Congressional, the Northern Irishman struggled all day.
He made eight bogeys, including three in his last four holes, and just one birdie to leave himself with a battle just to make the cut, let alone retain his title.
"I tried to approach it like any other tournament I play. I tried to go out there and shoot the best score I could," said McIlroy.
"Today wasn't my greatest day, but hopefully I can come out tomorrow morning and try and shoot a good one and at least try to be here for the weekend."
McIlroy's limp performance could not have been any more different to last year's, when he was a model of efficiency from tee to green and brimming with self-belief.
In 2011, he missed only 10 greens in regulation during the entire tournament but in his opening round on Thursday, he missed 12.
McIlroy was also erratic off the tee and hit the fairway seven of 14 times, cutting a forlorn figure as he trudged off at the last after yet another bogey.
"Too many times, I was just in the wrong position off the tee or with my second shot and it makes it very difficult," he said.
"When you're trying to play catch-up on this golf course it's very hard.
"You have to be so precise. Anything just a little off and it really punishes you."
McIlroy has struggled for consistency since he briefly climbed to the top of the world rankings earlier this year.
Memories of last year's Masters meltdown came flooding back when he finished tied for 40th at Augusta National in April and although he came second at Quail Hollow, the site of his first PGA Tour win in 2010, he missed the cut in his next two events.
The 23-year-old showed some encouraging signs of a timely return to form when he completed his US Open preparations with a tie for seventh at the St Jude Classic last week.
But even that was tinged with some disappointment after he led by two shots on the final day but made a double-bogey on the last hole when he needed a birdie to get into a play-off.
While McIlroy floundered, his countryman Graeme McDowell flourished in the difficult conditions, shooting a 69.
McDowell won the US Open at nearby Pebble Beach two years ago with a four-round total of even par and said McIlroy could not be counted out just because of one bad round.
"If anybody can come back from it, he can," McDowell said.
"He'll be coming out with guns blazing tomorrow trying to get himself back in the mix."
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