Thomas Detry one stroke ahead at Pebble Beach while Rory McIlroy pays for breaking the rules

McIlroy, playing his first PGA Tour round of the year after a win and a second place in Dubai, was leading after 14 holes as three successive birdies took him to six-under-par.

But the world number two dropped five strokes in his final four holes at Spyglass Hill – one of two courses being used by the 80-strong field – including a triple bogey at the par-five seventh hole after he fell foul of a rule change which cost him a two-shot penalty.

Having dropped within a club length of an unplayable lie, he was informed before signing his card for a final 71 of a change in the rules that a player must drop on the direct line.

"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 sort of changed, said McIlroy, who trails Detry by eight strokes.

The Belgian started and finished with three birdies as he carded an opening 63, one clear of American Patrick Cantlay.

Matthieu Pavon, who on Saturday became the first French winner on the PGA Tour since 1907, was a shot further back on 65 after the best round of the day at Pebble Beach.

Justin Rose birdied his three holes at Spyglass Hill in a four-under-par 68, one better than fellow Englishmen Tommy Fleetwood and Matt Fitzpatrick.

Nick Dunlap, playing his first round as a professional after winning The American Express tournament as an amateur last month, was tied for last place after a four-over-par 76.

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