"I got off to a fast start obviously," McIlroy told reporters. "I was three under through nine holes. I felt like I played really, really well on the front nine.
"Made a couple of bogeys on the back nine and that sort of halted the momentum. Overall it was good, to shoot under par today was a solid way to get off to any Major.
"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."
McIlroy won by eight shots at Kiawah Island last year.