AP photo by Julio Cortez / Adam Hadwin watches his shot on the 17th hole during the first round of the U.S. Open on Thursday in Brookline, Mass. Hadwin opened with a 4-under-par 66 for a one-stroke lead.

BROOKLINE, Mass. — Adam Hadwin can be excused for feeling like the opening round of the U.S. Open — annually billed as the toughest test in golf — gave him a chance to exhale.

Hadwin was home last week for his own national championship, an even bigger deal because the coronavirus pandemic had canceled the Canadian Open the previous two years and the golf-mad fans brought enormous energy to St. George's in Toronto.

Plus, he has been immune from the endless chatter and speculation regarding the LIV Golf series — the Saudi-backed league that has signed multiple players from the PGA Tour — that consumed attention all week at The Country Club ahead of the 122nd U.S. Open.

Small wonder he walked off Thursday with a 4-under-par 66 for his best score in 63 rounds at major championships and a one-shot lead over four-time major winner Rory McIlroy and four other players.

"Nice to get down here and a couple of days' rest and get going for this week," the 34-year-old 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."

No other major is more open — roughly half the 156-man field has to qualify — and it showed. Seven of the top 13 on the leaderboard 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.

Any lingering thoughts of the rival golf league came from McIlroy, though not because of anything he said. This time, his bold statement came a clean card and a few tough pars required at the U.S. Open. McIlroy didn't make a bogey until his final hole, when he missed the green and flung his club, a brief fit of anger that revealed as much desire as frustration.

The 33-year-old from Northern Ireland had a 67 that left him in the large chasing pack with four players, all of whom had to go through 36-hole qualifying: MJ Daffue of South Africa, Joel Dahmen of the United States, David Lingmerth of Sweden and Callum Tarren of England.

At the opposite end was six-time major champion Phil Mickelson, who played in the LIV's debut event last week and on Thursday 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 strokes, and he only dropped one 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.

McIlroy has become a leading voice on the PGA Tour over the past few years, particularly with his rebuke of the Saudi-funded series that is disrupting golf. Thursday was a reminder he's pretty good at his day job, too.

He made two straight birdies late in his round to become the first player to reach 4 under, only to miss the ninth green and make his only bogey. At the moment, McIlroy isn't concerned with his strong stance against LIV Golf.

"It's been eight years since I won a major," he said. "And I just want to get my hands on one again."

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AP photo by Charles Krupa / Rory McIlroy hits on the fifth hole during the first round of the U.S. Open on Thursday in Brookline, Mass.

Even with a good start and coming off a victory in the Canadian Open, it doesn't figure to be easy for McIlroy or anyone else. The Country Club might be as accommodating as it gets all week, with moderate wind and cloud cover keeping the sun from making greens crispy and firm — and the best anyone could do was a 66.

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

Luke List, trying to make the cut at a U.S. Open for the first time in his sixth try, opened with a 72 and was tied for 57th. He's one of two former Baylor School standouts in the field.

The other is Harris English. Fourth at the U.S. Open in 2020 and third last year, he started on the 10th hole Thursday and was tied for 79th after a 73 that included two birdies and five bogeys — including four in a row from the 15th to the 18th. English is playing a major for the first time since last year's British Open, having missed the Masters and the PGA Championship this spring while recovering from hip surgery.

For McIlroy, it was his second straight major — and third time in his last four U.S. Opens — he opened with a score under par. There was confidence in his game after winning Sunday in Toronto, and there was passion rare for a Thursday unless the game is going badly.

He tried to drive the reachable par-4 fifth hole and caught an awkward lie in the thick collar above a bunker, forcing him to stand in the sand. He hit that into another bunker, and then twice slammed the club into the sand out of frustration. But he managed to save par.

"You're going to encounter things at a U.S. Open, whether they be lies or stuff like that, that you just don't really encounter any other week," he said. "It's hard not to get frustrated because I'm walking up there going, 'Just come back into the bunker.' The thickest rough on the course is around the edges of the bunker. So I was sort of cursing the USGA whenever I was going up to the ball."

And then from the ninth fairway, his approach sailed to the right and he flung his club. He couldn't save par on that one and had to accept a 67 — not a bad start, and no apologies for his few outbursts of emotion.

"Almost to remind yourself sometimes how much it means to you," he said.

There's a lot on the table outside of golf, too, with 13 players at the U.S. Open who took part in the LIV opener near London, leading the PGA Tour to suspend those members.

McIlroy, the first to shut down talk of rival leagues in 2020, spoke passionately this week about building on the legacy handed down by Arnold Palmer and Jack Nicklaus. For those taking the guaranteed money for 54-hole, cut-free events, he said it felt like "the easy way out."

But now it's time for golf, and there a feeling of relief that focus could turn to a U.S. Open that first came to this course more than a century ago. Thursday was more about birdies and bogeys — mostly the latter in a U.S. Open — and a place in history.