McIlroy came into the event without a top-20 finish on the PGA Tour this year but shrugged off early-season struggles with his irons to share the clubhouse lead with Xander Schauffele.

The world No 2 mixed 10 birdies - his career best in one round at TPC Sawgrass - with a bogey and a double-bogey to post a seven-under 65 in calm conditions on Thursday morning.

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McIlroy's fast start was overshadowed by a lengthy debate between playing partners Spieth and Viktor Hovland at the seventh, with both querying where he was dropping his ball after finding the water.

Spieth suggested the ball may have crossed the red line at a different point, with the conversation lasting several minutes and a rules official called to bring an outcome.

"I think Jordan [Spieth] was just trying to make sure that I was doing the right thing," McIlroy said. "I mean, I was pretty sure that my ball had crossed where I was sort of dropping it. It's so hard, right, because there was no TV evidence.

"I was adamant. If anything, I was being conservative with it. I think at the end of the day we're all trying to protect ourselves, protect the field, as well.

"I was adamant, but I guess I started to doubt myself a little bit. I was like, 'OK, did I actually see what I thought I saw?'. It is a bit of a [television] blind spot. I think the best view was from the tee, which was the view that we had.

"I'm comfortable [with the decision], I think that's the most important thing. I feel like I'm one of the most conscientious golfers out here, so if I feel like I've done something wrong, it'll play on my conscience for the rest of the tournament.

"I'm a big believer in karma and if you do something wrong, I feel like it's going to come around and bite you at some point. I obviously don't try to do anything wrong out there, and play by the rules and do the right thing. I feel like I obviously did that those two drops."

McIlroy makes fast start at Sawgrass

Beginning on the 10th tee, McIlroy made the perfect start when he opened with a birdie and then two-putted from 60 feet to take advantage of the par-five 11th.

McIlroy followed a five-foot birdie at the 12th by rolling in from 15 feet at the 14th, with the Northern Irishman jumping clear of the field when he added a tap-in birdie at the par-five 16th.

He made another at the iconic par-three 17th to leave McIlroy with six birdies in the first eight holes for the first time in his PGA Tour career, only for him to find the water with an aggressive tee shot at the par-four 18th.

McIlroy limited the damage to a bogey to reach the turn in 31 and responded strongly with birdies at the second and fourth, with the 2019 champion joined on seven under when Schauffele made five birdies in a six-hole stretch from the 18th.

A 15-foot birdie at the par-four sixth edged McIlroy back ahead and put Tom Hoge's course record of 62 under threat, before his tee shot at the seventh finished in the hazard and led to the lengthy debate with Spieth and Hovland about where he should make his drop.

His next shot finished short of the green and led to a double-bogey six, although McIlroy bounced back to birdie the par-five ninth - his final hole of the day - to join Schauffele on seven under.

It had a little bit of everything," McIlroy told Sky Sports about his round. "I got off to a great start and my play was much improved compared to the last few weeks, which I've worked really hard on over the past three days.

"I felt like I'd figured something out, but you never know until you get into the heat of battle if it was able to carry over from the range onto the golf course."

Canada's Nick Taylor was a shot back after a six-under 66, while Jason Day, Hoge and Ludvig Åberg were among the group two off the pace after carding opening-round 67s.

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