McIlroy bounced back from a double bogey at the 10th hole, where he knocked his second shot over a fence and took a one-stroke penalty.
After a bogey at No. 11 dropped him to even on the day, McIlroy ripped off a string of four straight birdies to surge back up the leaderboard Thursday. He just missed an eagle putt at the 18th, tapping in for his fifth birdie in the final seven holes.
Coming off wins at the British Open and Firestone, McIlroy is just one stroke off the lead at Valhalla, trailing Lee Westwood, Kevin Chappell and Ryan Palmer.