The 37-year-old, bidding to clinch back-to-back majors after defending his Masters title at Augusta last month, was among the morning starters at Aronimink Golf Club in south-west Philadelphia.

McIlroy unravelled with five bogeys on his back nine to fall away from the clubhouse lead which stood at three under par.

Starting from the 10th hole, McIlroy made a troubled start with an opening bogey, after having to hack out of the rough, but bounced straight back with a birdie at the 11th.

N/A

From there it was a run of 10 straight pars for the Northern Irishman, who is chasing a seventh major title which would take him past Sir Nick Faldo’s tally and see him become the most successful European of the modern era.

His birdie putt on 18 fell agonisingly short as he bent double in anguish before he looked on in disbelief as his ball ran back to the front of the first as he made the turn at even par.

The two-time US PGA champion could not find the spark and he showed his frustration as a sloppy drive on the fourth resulted in a second bogey.

Again he hit straight back by sinking a 31-foot putt but it all fell apart from there, with inaccurate tee hosts leading to four bogeys on the spin which decimated his round.

McIlroy had huge support from the vast galleries following his eye-catching group, which also featured Jon Rahm and Jordan Spieth – the latter chasing a win here to complete the career grand slam.

Rahm bogeyed the opening 10th and was losing his battle with the daunting and vast greens. His approach to the first ran off the front, as a spectator bellowed: “Welcome to Aronimink, Jon!”

The Spaniard failed to get up and down as he made a second bogey, taking out his frustration by slamming a bottle into a bin on the second tee.

But things can turn in an instant and they did for Rahm, whose pitch from 98 yards took two hops and landed in the hole to make his first ever US PGA Championship eagle and draw him back to one under, which is where he finished.

It ignited the fans gathered around the second as Rahm, all smiles, plucked his ball out and tossed it into the galleries.

Spieth, whose last major win came at the 2017 Open, was the one making a move with three birdies on his back nine but two late bogeys left him at one under.

In the group ahead, 2024 winner Xander Schauffele made the early running by birdieing three of his first four holes before finishing at two under.

England’s Alex Fitzpatrick was among the first group to go and started with two birdies, reaching the 14th at two under.

N/A

But a run of double bogey, bogey, bogey damaged his round as he ended at two over.

Bryson DeChambeau, runner up in the last two years, endured a torrid round as he posted a six-over par 76.

South Africa’s Garrick Higgo made a one-under 69, impressive given he was penalised two shots for being late to the tee.

World number one and defending champion Scottie Scheffler starts his championship at 2.05pm local time (7.05pm BST) alongside Matt Fitzpatrick and Justin Rose.