McIlroy was one of many high-profile stars to struggle on the first day at Shinnecock Hills, scrapping his way to a 10-over par 80 before a poor start to his second round left him with too much to do to avoid an early exit.
But McIlroy spent his unscheduled weekend off working hard to iron out some minor flaws in his swing, and the results were evident as he fired a six-under 64 at TPC River Highlands to end the first day just one off the lead.
The world No 7 hit 17 greens in regulation and converted seven chances for birdies until making his lone mistake at the last, when his back foot appeared to slip on his downswing as he hit his approach.
McIlroy was unable to get up and down to save his par, but he was delighted that his extra practice time had paid off as he looks to head into a busy summer in Europe on the back of his second PGA Tour win of the season.
“I feel like the work that I did over the weekend has started to pay off already,” said McIlroy, who also fired a 64 in the final round last year. “It was nice to see the shots I was playing right there, and being able to work the ball both ways was something I wasn’t quite as comfortable doing last week.
“Obviously you never want to miss a cut in a major, but it might have been a blessing in disguise for the rest of the year,” added the 29-year-old, who is attempting to revert to the type of swing he had when he first started winning on both the European and PGA Tours.
“I’m trying to get back to the way I swung in sort of 2010, 2011, but it’s sort of hard because my body has changed quite a bit since then and the feelings I have,” he said. “But the feeling I have right now is the feeling I had in the middle of 2009.
“So it’s just trying to go back, and, okay, I was swinging it really well then, but what was I doing? What was I thinking about? What was the focus on the swing? I’m just trying to rack my brain to recreate feelings that I had back then.
“That’s basically what I did over the weekend. I got a feeling that really sort of resonated with me and brought me back to a time when I was swinging it really well, and just sort of went with that feeling.”
Jordan Spieth was also delighted with his response to missing the cut last week, carding six birdies and a superb eagle at the sixth when he holed out from a greenside bunker in similar style to the shot that won him the tournament last year.
The defending champion ended day one tied for the lead with Zach Johnson, and he said: “I’ve been sticking to the process. I’ve been very positive about making progress this year, so it’s good to see a good score.
“Those are obviously nice to see, but there’s still a long way to go. It’s round one, and the first rounds have been kind of detrimental to me, so it’s nice to be in the thick of things.
“First rounds have been tough for me and I’ve been trying to do a little bit too much, trying to get shots back when I drop one and trying to have to birdie easy holes. I just had played toward centre of greens today and let the rest take care of itself.
“The putter is starting to look better to me, so I can play a little bit more conservatively tee to green and still get a lot out of the round, and that’s nice to be back in that kind of condition.”