McIlroy matched Smith's 10 birdies in the final round of the Australian's victory two years ago, but also dunked two tee shots into the water during his lowest start ever at the PGA Tour's flagship event.

The Northern Irishman had a seven-under 65 to share the lead with US Open champion Wyndham Clark and fellow American Xander Schauffele.

Schauffele played bogey-free with a two nervous moments toward the end of his round, one leading to birdie and another for an unlikely par save.

Clark, already a winner at Pebble Beach this year, came on strong with three late birdies.

McIlroy would have led outright if he'd not pulled his tee shots into the water at the seventh and 18th holes.

"It would be nice to shoot 62 and not have two in the water," he said.

The drama came not from the shots, but where to take his penalty drop. There were questions from Jordan Spieth and Viktor Hovland on the 18th - their ninth hole of the day - but that was sorted out and McIlroy ripped a three wood onto the green and escaped with bogey.

It was on the seventh hole that Hovland and Spieth wanted some clarity on whether the tee shot was above or below the red hazard line. That would be the difference of having to drop back by the tee or where it went into the water. Television replay didn't make it entirely clear.

There was one tense moment when Spieth said, "Everyone I'm hearing that had eyes on it ... is saying they were 100 per cent certain it landed below the line."

"Who's everybody, Jordan?" replied McIlroy's caddie, Harry Diamond.

"Who are you talking about?" McIlroy added.

McIlroy was comfortable in saying it was above the line, played short of the green and wound up making double bogey.

"I think Jordan was just trying to make sure that I was doing the right thing," McIlroy said.

"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.

"But I think, again, he was just trying to make sure that I was going to do the right thing."

Spieth had his own issues and posted a 74, leaving him in danger of missing the cut at the Players for the sixth time in 10 years. Hovland took double bogey on his final hole for a 73.

The leading trio have a one-shot buffer over England's Matt Fitzpatrick and Canadian Nick Taylor.

Australian Jason Day is one stroke further back with world No.1 and defending champion Scottie Scheffler and five others.

Calm weather by Florida standards in March and a soft Stadium Course made the opening round perhaps as easy as it will get all week.

There also was a front pin on the island-green 17th, where New Zealand's Ryan Fox made a hole-in-one.

"It's such an iconic hole, and it's an intimidating shot," Fox said.

"I don't care who you are. You get up there, most of the crowd probably either wants you to make a one or hit it in the water, so I'm glad to be on the right side of it in that respect."

With AAP

Australian Associated Press