American Brandon Hagy sank a career-best 10 birdies in shooting an eight-under-par 64 to grab the lead after Thursday's opening round of the PGA American Express tournament.
World No 398 Hagy, seeking his first PGA title after missing the cut in five of his past six starts, took a one-stroke lead over South Korean An Byeong-hun.
Scotsman Martin Laird, American Max Homa and 2017 Players Championship winner Kim Si-woo of South Korea were on 66.
The tournament at PGA West in La Quinta, California, was played on the par-72 Nicklaus and Stadium courses.
Hagy, who played the Nicklaus layout, was only one off his PGA career low round score, set last year at La Quinta.
"I've been working on some great stuff the past couple of weeks," Hagy said. "Made those 10-to-15-foot-range birdie putts all day. That's what you have to do to make 10 birdies."
Hagy, 29, only made the field when world number two Jon Rahm of Spain withdrew but has made the most of his chance.
Hagy opened with a three-putt bogey, then birdied six of the last seven holes on the front nine, reeling off two stretches of three in a row with a par at the sixth.
He missed the green at the par-3 12th on his way to a bogey but then birdied the next three holes and closed out the round with an eight-foot birdie putt, grabbing the lead alone.
An, also playing the Nicklaus course, birdied the par-5 fourth and seventh with another birdie in between at the par-4 sixth. The world number 77 reeled off three birdies in a row starting at the par-5 11th and added a final birdie at the par-3 17th.
"I never thought about the score all day," An said. "It was one of those steady rounds and at the end you shoot 7-under par. Everything worked."
An, 29, seeks his first US PGA victory, although he captured the European Tour's BMW PGA Championship in 2015 at Wentworth.
An missed the cut in five of his past seven events, including the US Open and Masters.
"I was struggling with my long game," An said. "I've worked pretty hard on that during the off season. Obviously it's paying off."
Kim, a back-nine starter on the Stadium course, birdied the par-5 11th and par-3 13th and 17th holes, then sank a 30-foot eagle putt at the par-5 fifth and birdied the par-5 eighth from seven feet.
"I felt pretty comfortable," Kim said. "Last week, I didn't putt very well and I just focused on practicing harder and I putted pretty well."
American Maverick McNealy, in a pack on 67, joined Dean Wilson from the 2009 final round as the only players to eagle the par-4 sixth hole at the Nicklaus course, doing so in a birdie-birdie-eagle-birdie run.
Mark Wilson withdrew from the event Thursday after testing positive for Covid-19 before the round began. He joined fellow American DJ Trahan, who withdrew on Tuesday to begin a quarantine after testing positive.
Leading scores after Thursday's first round of the PGA American Express tournament at La Quinta, California (par-72, USA unless noted):
64 - Brandon Hagy
65 - An Byeong-hun (KOR)
66 - Kim Si-woo (KOR), Martin Laird (SCO), Max Homa
67 - Charl Schwartzel (RSA), Sam Ryder, Michael Thompson, Tyler Duncan, Doug Ghim, Maverick McNealy, Alex Noren (SWE), Andrew Putnam, Ben Martin
68 - Brian Harman, Aaron Baddeley (AUS), James Hahn, Josh Teater, Jamie Lovemark, Adam Long, Austin Cook, Anirban Lahiri (IND), Kevin Streelman, Adam Schenk, Lee Kyoung-hoon (KOR), Kristoffer Ventura (NOR), Chris Kirk, Rory Sabbatini (SVK), David Hearn (CAN), John Huh, Cameron Davis (AUS), Mark Hubbard, Patrick Reed, Im Sung-jae (KOR), Nick Taylor (CAN), Brendan Steele, Tony Finau, Cameron Tringale