Starting on the back nine, the Northern Irishman birdied five of the first eight holes. And a further birdie at the second ended a flawless round of 66 to leave McIlroy tied with Zach Johnson.
He said: “It’s my best score around this course by a long way. I adopted more of a conservative strategy off the tee this year. But once you put your ball in the fairway you can be more aggressive into the greens.”
McIlroy trails 27-year-old Roberto Castro by three shots after the American carded a course record-equalling 63.
Further back in the field, Tiger Woods was one of six players to shoot 67 and Masters champion Adam Scott finished on 69.