Brandon Hagy, a late entry in The American Express field, shot an 8-under-par 64 on Thursday to take the first-round lead in La Quinta, Calif.
Hagy replaced the injured Spain's Jon Rahm on Monday and made the most of his opportunity, carding 10 birdies -- six on the front nine -- for a one-shot lead over South Korea's Byeong Hun An.
"I felt like I saw the greens really well, and the last couple of days, I felt like I was putting really well," Hagy said. "Once I started to read them well, I knew if I could just keep doing what I was doing, I'd have a good shot to do something."
Max Homa, Scotland's Martin Laird and South Korea's Si Woo Kim all shot 66 to sit tied for third at 6 under while nine players are tied for sixth at 5 under. Hagy, An, Laird and Homa played on PGA West's Nicklaus Tournament Course; Kim played on PGA West's Stadium Course.
Hagy started his round with a bogey on No. 1 but rattled off three runs of three birdies in a row, capping his round with a birdie on No. 18. Hagy sat out all of 2018 due to a wrist ailment and was limited the following year by a back problem.
"I'm happy with what I've been doing," said Hagy, whose only top-10 result in 76 previous PGA Tour starts was a tie for fifth in the 2017 RBC Canadian Open. "I'm getting to a point now where I know exactly what I need to work on and to just keep getting the reps."
Hagy fell a stroke shy of his career-low round. He posted a 9-under 63 in Round 2 of last year's The American Express at La Quinta Country Club, which isn't part of the event rotation this year.
Rahm, the pre-tournament favorite, pulled out with what he said was a minor injury.
An posted a bogey-free round, carding seven birdies. He has missed five cuts in seven events to open this season, finishing no higher than a tie for 35th.
Patrick Cantlay, the betting favorite to win the tournament in Rahm's absence, posted a 3-under 69 at the Nicklaus Tournament Course, leaving him tied for 39th. Brooks Koepka shot a disappointing 72 to tie for 103rd out of the gate in his first start at The American Express.
Kevin Na, who won the Sony Open in Hawaii last week, opened with a 3-over 75 at the Stadium Course.
Other notables include Patrick Reed (68 at the Stadium Course), Scottie Scheffler (70 at the Stadium Course) and tournament host Phil Mickelson, sitting in a tie for 132nd after a 2-over 74 at the Nicklaus Tournament Course.
After golfers play each of the two courses the first two days, the top 65 and ties will advance to compete at the Stadium Course on both Saturday and Sunday.
--Field Level Media