OAKMONT, Pa. — It was a war of attrition.

It was a pillow fight.

It was a day filled with stress.

It was a mess.

And then J.J. Spaun emerged from the utter chaos, like a fairytale in the end, authoring one of the most unlikely finishes in the 125-year history of U.S. Opens on Sunday at mighty Oakmont.

Spaun is a 34-year-old embraceable underdog, a late bloomer who’ll now forever be known for birdieing the final two holes to seize his first major championship on a day when it looked like the tournament might bleed into a Monday playoff.

“It’s definitely like a storybook, fairytale ending, kind of underdog fighting back, not giving up, never quitting,’’ Spaun said. “With the rain and everything and then the putt, I mean, you couldn’t write a better story.’’

Ah yes, the putt.

Spaun delivered a Michael Jordan-Kobe Bryant walk-off 3-pointer moment, burying a 64-foot, five-inch birdie putt in the gloaming on the 72nd hole that closed the deal.

To put this thing into proper perspective, the longest putt Spaun had made in 16 PGA Tour tournaments this year before Sunday was 35 feet. On his way to a 3-under-par 32 on the back nine, he made 136 feet in putts on the final seven holes.

Oh yes, about the 32 on the back nine. It was the lowest score on the final nine holes of a U.S. Open since Tiger Woods in 2000 at Pebble Beach.

“To finish it off like that is just a dream,’’ Spaun said. “You watch other people do it. You see the Tiger chip [at the 2005 Masters], you see Nick Taylor’s putt [to win the 2023 Canadian Open], you see crazy moments. To have my own moment like that at this championship, I’ll never forget this moment for the rest of my life.’’

Neither will his fellow combatants.

While Spaun, even par at the time with a one-shot lead, was holing out that bomb on 18 to get to 1-under, Scotsman Robert MacIntyre, who’d just posted 1-over par, was sitting in the scoring room watching it on TV hoping for playoff.

When the putt went in, MacIntyre simply started clapping and said, “Wow.’’

A few yards away from 18, Tyrrell Hatton, who was also in contention until the end, was conducting an interview with reporters when he heard the roar on 18 for Spaun’s putt. He stopped in mid-sentence and said, “He’s holed it. What a putt to win. That’s incredible. Unbelievable. Fair play.’’

Near the 18th green was Spaun’s wife, Melody, with their two daughters, Violet and Emerson, and she kept repeating to them, “Daddy won, Daddy won.’’

“Daddy,’’ a journeyman who’s played on smaller tours such as the Gateway Tour, the Canadian Tour and the Korn Ferry Tour before making it to the big leagues, had won only one PGA Tour event in his career before Sunday — the 2022 Valero Texas Open.

It was his playoff loss to Rory McIlroy in the Players Championship in March, though, that injected the most belief in Spaun.

That allowed him to keep his wits about him as he took the first-round lead with a 65 on Thursday and was near the top of the leaderboard all week.

“There was no faking, there was no hiding since Thursday,’’ Spaun said. “That’s what I’ve been able to overcome. I’m not trying to shy away from the moment. I just tell myself, ‘If I can do this when there’s no pressure or no lead, like why can’t I do it when there is?’ ’’

That belief kept Spaun from going off the deep end Sunday, when he bogeyed five of his first six holes and looked like he was done in by a series of bad breaks.

No one among the cluster of players who were in the mix all afternoon in the quagmire that should have renamed Oakmont “Soakmont’’ with a 97-minute rain delay deserved to win more than Spaun.

He was enduring bad breaks that would have broken most players en route to shooting 40 on the front nine.

The worst came on No. 2, where he appeared to have stuffed his approach shot to within birdie territory, but the ball one-hopped off the flagstick and rolled 50 yards off the green and ended in bogey.

“After his start, it just looked like he was out of it immediately,’’ Viktor Hovland, Spaun’s playing partner, said.

Hovland had his part in this drama, too.

After Spaun birdied the short par-4 17th after driving the green to take the lead at even par, then split the fairway with his drive on 18 into the fairway and hit 6-iron onto the green, Hovland hit his approach shot just outside of Spaun’s ball.

After all the bad breaks Spaun had endured earlier, this was a deserved gift. Hovland was going to give Spaun a read on his birdie putt.

Hovland nearly holed his putt, giving Spaun an amazing look, and then Spaun stepped up and buried the birdie to seal victory, sending the packed greenside galleries into a state of bedlam.

“The [putt] on 18, it’s just absolutely filthy there,’’ Hovland said.

“Viktor helped me a lot,’’ Spaun said. “It was a foot left of my line. I was just in shock, disbelief that it went in and it was over.’’