World number one McIlroy made an uncharacteristic slow start to the FedEx Cup play-offs opener on Thursday, with a double bogey and three bogeys seeing him end the day nine shots off the pace at the Ridgewood Country Club in New Jersey.
McIlroy openly admitted he did not prepare adequately after basking in the glory of his three successive triumphs at The Open Championship, WGC-Bridgestone Invitational and the US PGA Championship.
But the Northern Irishman recaptured his top form in Friday's second round, going bogey-free and holing six birdies for a 65 to be within five shots of co-leaders Adam Scott and Cameron Tringale at the halfway stage.
"I went to the range yesterday afternoon, worked on a couple of things and just got comfortable with my swing again," McIlroy told Sky Sports.
"After not really touching a club before this week I felt a little bit rusty out there yesterday. It didn't take long, maybe an hour on the range to sort of get things back on track, and I'm glad I did it because I played really well today.
"I'm going to have to get off to a fast start tomorrow to try and catch those guys before they go out on the golf course."