Adam Hadwin of Canada fired a 4-under-par 66 late on Thursday to secure the first-round lead in the US Open at The Country Club in Brookline, Massachusetts.
Hadwin moved ahead of Rory McIlroy, who shared the lead at the conclusion of his round before ending up in a five-way tie for second place at 3-under 67.
McIlroy, a four-time major champion from Northern Ireland, was tied with Joel Dahmen, England's Callum Tarren, David Lingmerth of Sweden and MJ Daffue of SA. England's Matt Fitzpatrick and Justin Rose, as well as Dustin Johnson, Hayden Buckley, Matthew NeSmith, Brian Harman and Aaron Wise were tied for seventh at 2-under 68.
Hadwin compiled five birdies in a six-hole stretch to make the turn at 4-under 31. That included three straight birdies at numbers 7 to 9, with his second shot at the par-4 ninth rolling to inside three feet of the pin.
After a bogey at the par-4 12th, Hadwin recovered with a birdie at the par-4 13th to re-establish himself as the sole leader.
Hadwin, 34, earned his only PGA Tour victory in 2017. He has never finished inside the top 20 at a major.
McIlroy entered as the odds-on US Open favorite after he won the RBC Canadian Open on Sunday. He stayed hot, starting his day on the back nine with two birdies before holing back-to-back birdies at the seventh and eighth holes to reach 4 under.
But a tough shot from off the ninth green left him a long par putt he couldn't convert, and his only bogey of the round left him out of the outright lead at the end of the day.
"I wanted to try to shoot a round of golf on a really tough course without making a bogey," McIlroy said on the NBC broadcast.
"I achieved that for 17 holes and then didn't quite save par there at the last. But overall a really good day. Had some really good par savers. Hit the ball pretty well and it's a great start to the tournament."