Sam Burns notched two eagles on the way to an 11-under-par 61 on Friday to take a one-stroke lead over Michael Kim in the US PGA Tour American Express tournament in La Quinta, California,
The 27-year-old American said he didn't realise he was within sight of the 13th sub-60 round in PGA Tour history until he was on the 17th green, where his birdie putt from about 25 feet slid by.
He parred 18 to cap his career-best round, heading the field on 17-under-par 127 on another low-scoring day in the California desert east of Los Angeles.
Burns kickstarted his round with a 15-foot eagle at the par-five fourth hole on the Nicklaus Tournament Course, one of three in use over the first three rounds of the event.
He added birdies at the fifth, sixth and seventh, then eagled the par-five 11th where he hit a "perfect seven-iron" that left him three feet for eagle.
"Overall just a really solid day," said Burns, who added birdies at 13, 14, 15 and 16 to grab the solo lead. "I hit a lot of quality shots and was able to make some putts."
Burns said he wasn't thinking about a 59.
"Based off the scores, obviously everybody's shooting some good numbers, so I think (I was) just trying to keep pace and trying to hit as many quality shots and get as many looks as possible," he said.
Korean-born American Kim had 10 birdies and a bogey in his nine-under-par 63 for 128 on the Nicklaus Tournament course, where South Korean Lee Kyoung-hoon had nine birdies in his eight-under 64.
Lee was tied for third on 15-under 129 with amateur Nick Dunlap, a University of Alabama sophomore who shot an impressive seven-under 65 on the Stadium Course, the toughest of the three that will be used for Sunday's final round.
Dunlap got off to a fast start with six birdies on the front nine. His seventh came at the 18th, where he rolled in a six-footer for his share of third place.
Dunlap, the reigning US Amateur champion, is trying to become just the eighth amateur to win on the US PGA Tour since 1945, and the first since Phil Mickelson won in Arizona in 1991.
Dunlap said his strong start to the week came down to "trusting myself," in his fourth tour start.
"I've been playing some good golf recently and just tried to stay in that same mindset," he said. "Obviously, it's going to be a kind of a shoot-out, it seems like, so pedal to the metal and try to make some putts."
World No 6 Patrick Cantlay headed a group on 130 that also included 2021 champion Kim Si-woo of South Korea, South African Christiaan Bezuidenhout, American Eric Cole, Sweden's Alex Noren and Canadian Adam Hadwin – who carded a 59 at La Quinta Country Club in 2017.