Ludvig Aberg shot an opening nine-under-par 63 to take a two-shot lead in the Farmers Insurance Open.
The Swedish Ryder Cup star leads American duo Danny Walker and Hayden Springer.
Despite recording birdies on his opening two holes, Shane Lowry finished on a level par 72 having opened his tournament on the more difficult South Course - one of two being used for the tournament - to lie tied-87th overall.
Aberg began with three birdies on the North Course and added five more alongside an eagle at his eighth hole as he hit 16 of 18 greens in regulation.
The only blemish on his card came on his 11th hole when he dropped a shot after failing to find the fairway off the tee.
Walker was the only player who opened up on the South Course to be within four strokes of Aberg, alongside only two others in the top 18.
He hit every green in regulation, cashing in with seven birdies to join Springer who eagled his opening hole.
Among a group on six-under-par was former American Ryder Cup captain Zach Johnson, seeking his first win since The Open at St Andrews in 2015.
Lowry, making his first start of the season, had a challenging day on the South Course, the layout which hosted the US Open in 2008 and 2021, hitting five birdies and five bogeys en route to an even par round.
After a strong start, the 2019 Open champion was in the black at the turn after bogeys at the fourth, sixth and ninth. On the back nine, he continued to take one step forward and one back, with three birdies and a couple of dropped strokes but did conclude on a high note, sinking a 7ft putt for birdie at the par-5 18th.
With additional reporting: PA
