The four-time major winner from Northern Ireland also birdied his final hole to move ahead of Will Zalatoris and Tom Hoge, who shot 4-under 66 during the early wave of golfers. Matt Kuchar and Mexico's Abraham Ancer were tied for fourth at 3-under 67.
Playing in a group with Tiger Woods and Jordan Spieth, McIlroy started on the back nine and dialed up birdies at the 12th through 15th holes. At the par-4 12th, he stuck his second shot inside 2 feet of the pin for a tap-in. Another highlight came at the par-3 14th, where he rolled in a right-to-left birdie putt from 26 feet.
McIlroy added birdies at Nos. 2 and 5 to reach 6 under before giving two shots back with bogeys at two par-3 holes, the sixth and eighth. But at the par-4 ninth, he sank a 19-foot, downhill birdie putt to move back into sole possession of the lead.
McIlroy is a two-time PGA champion (2012, 2014), but since his title in 2014, he has yet to break through and win another major.
Woods started with two birdies in his first five holes before coming apart. He favored his surgically repaired right leg throughout the afternoon. He finished at 4-over 74 after making seven bogeys and one more birdie. Spieth shot a 2-over 72.
Zalatoris counteracted two bogeys with six birdies, including a strong finish with birdies at the seventh and ninth holes. Last season's PGA Tour Rookie of the Year tied for eighth at the 2021 PGA Championship, shortly after he took second place in his Masters debut.
Hoge collected five birdies and just one bogey. He opened with a 15-foot birdie putt at the par-4 first and had three more birdies inside 6 feet thanks to his approach game. The 32-year-old earned his first career PGA Tour win earlier this year at the AT&T Pebble Beach Pro-Am.
Notables who had just begun their round in the afternoon wave included the top three players in the world ranking — Scottie Scheffler, Jon Rahm of Spain and Collin Morikawa — plus Dustin Johnson, Patrick Cantlay, Justin Thomas and Brooks Koepka.