Canada's Adam Hadwin birdied five of the last six holes on the front nine and seized a one-stroke lead after Thursday's first round of the US Open with Rory McIlroy in a pack one adrift.

World number 105 Hadwin, whose best finish in 19 prior major starts was a share of 24th at the 2018 Masters, fired a six-under par 66 at The Country Club.

"Not much better of a start to a US Open you can ask for," Hadwin said. "To take the first round lead at the US Open is incredible. It's something no one can take away from me. But it's not the final goal."

Four-time major winner McIlroy, who tossed one club and twice swung another at sand in frustration, shared second on 67 after a closing bogey with Sweden's David Lingmerth, England's Callum Tarren, American Joel Dahmen and South Africa's M.J. Daffue.

Hadwin, whose only PGA title was at the 2017 Valspar Championship, missed a six-foot par putt to bogey the third hole, then birdied five of the next six holes, dropping his approaches between two and four feet on all but the par-3 sixth and making each short birdie putt to grab the outright lead.

"I kind of held down the fort there the last little bit," Hadwin said. "Made a nice one from the fringe on 13. I hit the ball really well the last four holes."

The 34-year-old Canadian missed the green at 12 on the way to a bogey but holed out for birdie at 13 from the rough from 36 feet to reclaim the lead.

"There are still 54 holes," Hadwin said. "There's a lot of golf left and this course is only going to get tougher."

For much of the day, third-ranked McIlroy shared the top spot despite his troubles, grinding out a solid start.

"Even though I'm standing up here slightly frustrated that I bogeyed the last, it's a great start," said McIlroy.

"You feel like you're right in the tournament from the start of the week, which is nice. It's certainly a different mindset when you get off to a good start. I've just got to keep it going."

Back-nine starter McIlroy rolled in four birdie putts between 10 and 25 feet and made several long clutch par-saving putts, notably at the fifth from 14 feet after failing to escape a bunker and smashing sand with his club.

"It's hard not to get frustrated," McIlroy said. "I gave the sand a couple of whacks because I'd already messed it up... it was really nice to hole that putt."

McIlroy, who won last week's PGA Canadian Open, has not captured a major title since the 2014 PGA Championship.

"It has been eight years since I won a major and I just want to get my hands on one again," McIlroy said.

Two-time major winner Dustin Johnson, among the rebels who left the US PGA Tour for the Saudi-backed LIV Golf Series, led a seventh-place pack on 68 that included fellow Americans Hayden Buckley, Matthew NeSmith, Aaron Wise, Brian Harman plus England's Matthew Fitzpatrick and Justin Rose.

In the first event to match PGA Tour and LIV Golf players, 15 LIV Golf signees combined to shoot 53-over par. Notables included Patrick Reed on 70, Bryson DeChambeau on 71, Sergio Garcia on 74, Louis Oosthuizen on 77 and Phil Mickelson with 78 on his 52nd birthday.

The US Golf Association chose not to follow a PGA Tour ban of 17 golfers who played in last week's LIV Golf debut in England, staying faithful to this year's US Open qualifying standards.

Tarren, a 445th-ranked journeyman, eagled the par-5 eighth, his penultimate hole, to leap into the hunt.

"I'm kind of pinching myself," Tarren said. "Just excited with my start."

Lingmerth, an alternate until injured Martin Kaymer withdrew Saturday, birdied 16 and 18 from just inside 30 feet and sank a birdie putt at the fifth from just under 18 feet.

"Got off to one of those starts," Lingmerth said. "I was never really in big trouble, but I putted really well."

Two-time US Open champion Retief Goosen has bolstered compatriot Daffue.

"He has been really sending me encouraging messages as far as the ability is there," Daffue said. "I can't believe how relaxed I was out there." (AFP)