Six-time major champion Phil Mickelson expects the Los Angeles Country Club to be a different animal on Friday after the first-time US Open venue yielded a pair of historic 62s in the first round.
Rickie Fowler and Xander Schauffle both carded US Open low rounds with their stunning eight-under par efforts to lead two-time major winner Dustin Johnson and Wyndham Clark by two strokes.
Fowler had 10 birdies as he broke the record of 63 first set by Johnny Miller 50 years ago and Schauffle matched him with a bogey-free round less than half an hour later.
The scoring average of 71.38 was the lowest ever in a US Open first round, and the sixth-lowest first-round average in major championship history.
Four-time major-winner Rory McIlroy and American Brian Harman both carded 65s as 37 players broke par and two players -- Matthieu Pavon and Sam Burns -- aced the par-three 15th.
Mickelson, who turns 53 on Friday as he tries yet again to complete a career Grand Slam with a US Open victory, said the gentle welcome to LA Country Club, a jewel of a course that members have largely preferred to keep to themselves, wouldn't last.
"I thought the course was incredibly set up," Mickelson said. "They moved some tees up and they had some soft pins to let us get off to a good start, but it'll play a lot harder as it goes on," Mickelson predicted.
"There's a lot more teeth in this golf course if they want to use it, and still it's fair."
- 'A little spicier' -
Misty morning conditions and a cloud layer that hovered all day kept the course from drying out and firming up.
While the US Golf Association is notorious for protecting par at the US Open -- and has in Opens past come in for criticism for course set-ups in the past, chief championships officer John Bodenhamer vowed organizers wouldn't make it "stupid hard."
"You'll see some things we had planned for Friday anyway and if we get the conditions we're hoping for, I think you'll see a little a little different, a little spicier golf course," he told Golf Channel on Thursday night.
"I'll tell you what we won't do, and I feel very strongly about this – we're not going to force anything," Bodenhamer added. "We could do things that would make it stupid hard."
The second round teed off under overcast skies, and Clark opened with a birdie at the 10th that moved him to seven-under.
McIlroy teed off at 8:24 alongside last month's PGA Championship winner Brooks Koepka -- who couldn't get in on Thursday's scoring festival and carded an opening 71.
McIlroy will be aiming to shake off a bogey at the 18th, where he whiffed on a shot out of the rough. It was a disappointing end to a fine round that saw the Northern Ireland star birdie five of the first eight holes and hit all but two greens in regulation.
Fowler was due to tee off at 1:32 pm (2032 GMT) with Schauffele going off at 1:54.
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