Davis Thompson produced back-to-back eagles on his way to a two-stroke lead after opening with a 10-under 62 at the American Express in California on Thursday.
The unheralded 23-year-old American leads the way from a stacked field that includes 10 of the top 20 in the world.
Thompson set up his career-low round by shooting an eight-under-par 28 on his front nine, including eagles on both the fifth and sixth holes at the La Quinta Country Club, which is one of three courses being used as players also spread across the PGA West's Stadium Course and Nicklaus Tournament Course.
The American's performance tied the lowest front-nine score on the course, having made six of seven fairways and eight of nine greens.
Behind Thompson is a group of five players, including former world number one Jon Rahm who carded nine birdies and a bogey in his round of 64 on La Quinta. Rahm is tied with Sam Burns, Tyler Duncan, Matti Schmid and Taylor Montgomery at eight under.
Burns might have been outright second if not for a bogey on the 18th on the Stadium Course, having carded seven birdies on his front nine. The American made nine consecutive one-putts.
Nine players are a further stroke back at seven under after the first round, including Tokyo Olympics gold medalist and two-time major runner-up Xander Schauffele, who also shot an eagle on the fifth.
Among the pre-tournament favourites, Scottie Scheffler and Patrick Cantlay both shot four under on La Quinta, while Will Zalatoris managed three under on the Nicklaus Tournament Course. Tony Finau and Tom Kim carded rounds of three under at La Quinta.