Adam Hadwin can be excused for the opening round of the U.S. Open -- the so-called toughest test in golf -- giving him a chance to exhale.

Hadwin was home in Canada last week for his own national open, an even bigger deal because the pandemic had canceled the Canadian Open the previous two years and the golf-obsessed fans brought enormous energy.

Plus, he has been immune from the endless chatter and speculation of the Saudi-backed rival league that consumed attention all week at The Country Club.

Small wonder he walked off Thursday with a 4-under 66 for his best score in 63 rounds at major championships and a 1-shot lead over Rory McIlroy and four others at 67: Callum Tarren of England, David Lingmerth of Sweden, MJ Daffue of South Africa and Joel Dahmen.

"Nice to get down here and a couple of days' rest and get going for this week," Hadwin said. "Not that the golf course gets any easier. But yeah, in certain instances it definitely felt a little more relaxed than last week."

At the opposite end was Phil Mickelson, who celebrated his 52nd birthday -- on the golf course, anyway -- with a four-putt double bogey on his way to a 78.

Hadwin ran off three straight birdies to finish the front nine in 31, and he dropped only 1 shot on the back nine for his 66.

His previous low score in a major was 68 on three occasions, most recently the first round of the 2020 PGA Championship at Harding Park.

The group at 68 included two-time major winner Dustin Johnson, Justin Rose and Matt Fitzpatrick, who won the U.S. Amateur at Brookline in 2013.

No other major is more open -- roughly half the 156-man field has to qualify -- and it showed. Seven of the top 13 came through qualifiers, including Hadwin.

He was first alternate out of the Dallas section and got in when Paul Casey withdrew because of an ailing back.