McIlroy blamed poor putting for his failure to record a single birdie after four bogeys left him nine shots behind clubhouse leader Jimmy Walker, with England’s Ross Fisher and Argentina’s Emiliano Grillo a shot off the summit.
“Tee to green is not the problem, but when I get to the greens it’s a different story,” said McIlroy, 27.
“I had a couple of chances early on that I could not convert and then missed a couple of greens and did not get up and down and I was always chasing it from there.
“I really struggled with the pace. They look much quicker than they are and two or three times I had putts within 12ft that were downhill and left them short.
“I need to be more aggressive with my stroke, figure it out tomorrow, and shoot something in the mid-60s and get into the weekend. That’s the first objective. I will stay as patient as I can and try to turn it around.”
McIlroy lifted the Wanamaker Trophy in 2012 and 2014 and has recorded three other top-eight finishes in seven appearances in the final major of the year.
The world number four was optimistic of turning what he called a B- season into an A+ by claiming his fifth major title on Sunday, but missed from 10ft for birdie on the 10th — his opening hole — and even closer range on the 11th, before dropping shots on the 13th and 14th.
A poor tee shot into a greenside bunker on the 16th led to a third bogey of the day and McIlroy was frustrated not to take advantage of the 17th and 18th, the only par fives on the par-70 layout.
McIlroy’s 77 on day three of the Masters in April was his first without a birdie in 80 major championship rounds and another followed as he followed a three-putt bogey on the second by lipping out from four feet on the sixth.
In contrast, Walker had carded six birdies and a solitary bogey in his 65, with Fisher completing his round with birdies on the 17th and 18th and compatriot Andy Sullivan picking up three shots in the last four holes to finish a shot further back.
“I feel like I’ve prepared and I’m ready to go this week,” Walker said. “It’s nice that that’s what showed today. I felt like I was ready to go. Winning a major would be huge but there’s three days to go.
“I had not been making a lot of putts this year. Last weekend and today I felt like I was gaining some momentum and starting to see some go in. So it was nice to keep that rolling today.”
Defending champion Jason Day believes his lack of preparation for the PGA Championship could prove a blessing in disguise after an encouraging opening 68.
Day only managed one practice round at Baltusrol after catching a bug from his son Dash and then spending Tuesday night in hospital after his wife suffered an allergic reaction.
The world number one admitted he was “running on empty” after defending his Canadian Open title immediately after finishing 22nd in the Open at Troon, but there were few signs of that as he carded three birdies and a solitary bogey yesterday.
“I drove it very nicely and hit a lot of good iron shots,” said the 28-year-old Australian. “I just didn’t capitalise on the opportunities that I had out there but for the most part I feel pretty good about where my head is at and looking forward to the next three days.
“By Thursday, if you practise Monday, Tuesday, Wednesday, you could be pretty tired with how hot it was. It was in the mid to high 90s and you can get pretty drained pretty quickly. I’m glad I took Monday, Tuesday off, even though I’m a little bit under the weather.
“It was kind of a blessing really to come out yesterday, play 18 holes real quick, and then get out of here.”
Day could lose his position at the top of the world rankings if he finishes lower than 28th and US Open champion Dustin Johnson is outright second or better, but added: “I know that the only way to stay at number one is to win tournaments, finish well in big tournaments.
“If I can keep focusing on the process, all the hard work that I’ve done before to get to this point, that’s where I need to keep my focus at. I’ve always said that winning takes care of everything.”
Fisher, whose sole top-10 finish in a major to date came in the 2009 US Open, said: “I struck the ball well all day, especially from the tee, and picking up birdies on 17 and 18 was obviously a lovely way to finish.
“I gave myself a lot of chances and missed very makeable putts for birdie on one and five. But four under to start a major, and with only one bogey on the card, I’ve obviously got to be very happy with that.
“My coach (Denis Pugh) has been banging on to me about keeping my swing nice and smooth, so I had that running through my head today and it’s paid off very nicely.”
Sullivan, who is on course to secure his Ryder Cup debut this season, said: “I’m buzzing with that start. I’ve been playing really well leading up to the tournament, so it’s just nice to keep the form and momentum going.
“It was a great atmosphere out there, the fans were making loads of noise, and it was nice to be playing in the group behind Rory, Jason (Day) and Phil (Mickelson), because you could get a good view of the crowds.
“It was a bit of a struggle at times in the heat, especially down the stretch there, so I’m happy to get finished and enjoy some nice air conditioning. It was seriously hot and humid even first thing this morning, so I was seriously melting out there. But I’d take this over the cold and wet we had at the Open any day, so I won’t complain” added Sullivan.