McCarthy carded 10 birdies in a stunning bogey-free 60 on the par-70 layout at TPC River Highlands, the venue which saw Jim Furyk's historic '58 round' in 2016, to top the leaderboard ahead of major winners Adam Scott and Keegan Bradley.

McIlroy is alongside Xander Schauffele and US Open champion Wyndham Clark on two under, with the former world No 1 posting a hole-in-one at the par-three eighth, alongside five birdies and five bogeys during an eventful first round.

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McCarthy opened with four straight birdies and two-putted from 60 feet to pick up another at the 15th, having driven the par-four green, then started his second nine with a run of three consecutive birdies.

The American got up and down from off the sixth green to add a ninth birdie of the day and move into the solo lead, before nearly holing his approach into the final green to set up a closing birdie.

Scott made four birdies in a five-hole stretch from the second and cancelled out a bogey at the seventh by picking up a shot at the ninth, seeing him turn in 31, then charged up the leaderboard by firing five consecutive birdies from the 11th.

The 2013 Masters champion saw his hopes of a sub-60 round ended when he found the water with his approach into the par-five 17th, although the Australian birdied the last to join Bradley on eight under.

Bradley had threatened a historic round himself when - beginning on the back nine - he birdied his first five holes and added three more in a five-hole stretch from the 17th, with the 2011 PGA champion cancelling out a bogey at the fifth by draining a 20-footer at the seventh.

Scottie Scheffler's opening-round 63, which put him in fourth place, included four birdies in the last six holes, while Shane Lowry is within four of the early lead after also making a blemish-free start in his 64.

McIlroy's ace part of mixed start

McIlroy rolled in a 10-foot birdie at the second but missed a chance from half the distance at the next, with the world No 3 bogeying his next two holes and slipping over par until his moment of brilliance at the par-three eighth.

"Five-iron to 215 yards, slightly back into the wind off the right," McIlroy explained. "As hole-in-ones go, I've had them in the past where it has been an okay shot and you get a little lucky.

"That was the best shot of the day that I hit. Obviously a bonus for it to go in the hole, but it was really cool. I played pretty averagely up until that point and felt like that hole-in-one got me going a little bit."

McIlroy almost holed his next approach from a bunker to set up a tap-in birdie, the first of back-to-back gains around the turn, only to bogey the par-five 13th after finding water off the tee.

He responded with successive birdies from the 14th, before two bogeys in his last three holes saw him card a 68 and fall eight strokes behind McCarthy at the top of the leaderboard.

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