Rickie Fowler and Xander Schauffele shared the lead after a record-setting start to the US Open on Thursday at Los Angeles Country Club where world number one Scottie Scheffler was among the closest pursuers in the clubhouse five shots back.
World number three Rory McIlroy went out late and was three shots back when he made the turn after covering his front nine in five under par, which left him level with Brian Harman, who was playing one group ahead of the Northern Irishman.
Fowler and world number six Schauffele fired matching eight-under-par 62s that marked the lowest round in US Open history and equalled the lowest major round of all time.
A resurgent Fowler, making his first US Open start since 2020, poured in a tournament-record 10 birdies to set the early pace and then waited to greet Schauffele, who was playing two groups behind, when the fellow American matched his score.
"I knew there was birdies to be made out here, but you have to drive it well and get the ball in position first," Fowler said. "Yeah, did that, and from there just managed our way around really well."
The 34-year-old American wasted little time making his move as he birdied his opening hole, the par-four 10th, and reached the turn in three-under 32 before a flawless trip through his closing nine holes during which he carded five birdies.
Schauffele, who has five top-10 finishes in his six US Open starts, also got off to a flying start with three birdies across his first five holes while avoiding a bogey all day at an event that prides itself as being the toughest test in golf.
"It's not really what you expect playing a US Open. But monkey see, monkey do," Schauffele said. "Was just chasing Rickie (Fowler) up the leaderboard. Glad he was just in front of me."
Pre-tournament favourite Scheffler, former champion Bryson DeChambeau of LIV Golf, South Korea's Kim Si-woo and lowly-ranked Frenchman Paul Barjon were all in the clubhouse five shots back of the co-leaders.
"The scores were a bit lower than I imagined them to be, but going out early on Thursday it's the easiest conditions you're going to see most of the week," said Scheffler, whose only two bogeys bookended his round.
"Hit a lot of good shots today and I did a good job of staying patient out there and posted a good number on a day in which I got off to a pretty slow start."
Masters champion Jon Rahm, looking to become the first player to win five times in a season since Justin Thomas in 2016-17, also went out early and carded a one-under 69 after mixing three birdies with a pair of bogeys.
Viktor Hovland, the only player to finish in the top 10 in each of the last three majors and in his first start since his Memorial triumph 11 days ago, opened with a one under 69.
Among the other notable late starters, PGA Championship winner Brooks Koepka was one over through nine holes while defending US Open champion Matt Fitzpatrick was one over through 10 holes.
As the US Open got underway, a report surfaced that the US Justice Department would review the PGA Tour's plan to form a unified commercial entity with Saudi Arabia's Public Investment Fund, which controls LIV Golf.