On a surprising day of weather when umbrellas gave way to sunglasses, Cantlay saved par five times and had eight birdies at Spyglass Hill for an eight-under 64 that left him one shot behind Thomas Detry.

Belgian Detry worked his own short-game magic at the end, chipping in from thick, damp rough for birdie on the 18th at Spyglass to finish with three straight birdies and a 63.

Torrey Pines winner Matthieu Pavon had the best round at Pebble Beach, closing with four birdies over the last five holes for a 65.

Rory McIlroy, no longer on the PGA Tour board but offering eye-opening comments this week that LIV Golf players should be able to return without punishment, was among the leaders until it all fell apart at the end at Spyglass.

He was leading at six-under, coming off five birdies in seven holes, when he three-putted for bogey. And then on the par-5 7th, he drove into trouble and took a penalty drop by going some 20 yards back on his line and then moving one club length to the right.

One problem – that rule was changed to allow that in 2019 – and then changed back in 2023. McIlroy was supposed to drop on the line between his ball and the hole. So, the two-shot penalty turned his bogey into a triple bogey, and he shot 71.

"I wasn't aware that that rule was changed again in 2023, so I took a drop thinking of the 2019 rules when everything was changed, not knowing that the rule was changed again in 2023," McIlroy said. "So got a two-stroke penalty there."

The 80-man field – the strongest and smallest for Pebble Beach, which is now a signature event offering a $US20m ($A30m) purse – was mostly happy they weren't drenched from a forecast that suggested even more rain on top of the 1.5 inches that dropped overnight.

Patrick Cantlay finds himself in a great position after the opening round of the AT&T Pebble Beach Pro-Am. PHOTO: Getty Images.

Detry had 10 birdies by keeping the ball in play off the tee, key on a week where players are able to lift, clean and place their golf balls in the short grass. The chip-in certainly helped.

"I felt pretty comfortable I would put it within 3 feet, to be honest," Detry said. "And it rolled nicely, just trickled in the hole. It was lovely to watch."

Cantlay has been a central figure as one of six player directors on the board who has worked through the June 6 agreement with Saudi Arabia's national wealth fund, then entertaining bids from private investors, selecting a Strategic Sports Group and then finalising a deal Tuesday night for a $US3 billion ($A4.5b) investment that includes equity ownership for players.

"I think you have to compartmentalise," Cantlay said.

"When you're doing what you're doing you have to be as focused as you can be. Maybe it's a little harder with all the distractions, but resolve to do as best I can in focusing at the task at hand."

The soft conditions and limited wind allowed for good scoring on both courses, with only 19 players who were over par.

Jason Day was the best of the Australians with his three-under 69 at Spyglass.

The former World No.1 is tied 14th and is one ahead of compatriot Cam Davis. Adam Scott opened with an even-par 72.