Dustin Johnson reacts to an approach shot on the 15th hole during the final round of the U.S. Open on Sunday.

Brooks Koepka holds up his trophy on Sunday after winning the U.S. Open in Southampton, N.Y. He is the seventh player to win the championship in back-to-back years, and the first since 1989.

Brooks Koepka hits an approach shot on the 12th hole Sunday during the final round of the U.S. Open Golf Championship in Southampton, N.Y.

SOUTHAMPTON, N.Y. • Hours before he headed out to Shinnecock Hills to defend his U.S. Open title in the final round Sunday, Brooks Koepka and his buddies had a bench-pressing contest at a local gym.

“He put up 225 (pounds) 14 times on a Sunday; that’s pretty impressive,” said high school pal Dan Gambill. “I was like, ‘What are you, nuts?’ We had a bet. His trainer said he couldn’t do 15, and he fell short on the 15th.”

No matter. Koepka made up for it. At the end of the day, he hoisted the 8.5-pound U.S. Open trophy after shooting a two-under-par 68 to beat Tommy Fleetwood by a stroke and become the seventh player to win the storied championship in back-to-back years.

“Probably couldn’t have dreamed of it in my wildest dreams,” said Koepka, 28, the tournament’s first repeat winner since Curtis Strange in 1989. “I’m at a loss for words right now, but it’s really incredible.”

Koepka held off his good friend and closing-round playing partner, Dustin Johnson, who had a commanding four-stroke lead after two rounds before stumbling Saturday and shooting even par Sunday. Johnson finished third at three over.

For Koepka, it turned out to be an unforgettable Father’s Day present for his dad in the form of a second major championship. Bob Koepka didn’t attend the U.S. Open last year because he couldn’t find a convenient place to stay (and his son wasn’t expected to win).

“Two years in a row, I haven’t gotten him anything,” the younger Koepka said with a laugh. “Next year, I’m not going to get him anything, either.”

After an embarrassing Saturday for the U.S. Golf Association, when the greens were so dry and fast that players complained they were unfair, groundskeepers watered the putting surfaces to slow them and make them easier to hold. The average scores dropped accordingly, from 75.328 on Saturday to 72.179 on Sunday.

Fleetwood, who finished two hours before Koepka, watched from the clubhouse after shooting a 63, which tied the record for the lowest round in a U.S. Open.

“Yeah, but I wanted 62,” said Fleetwood, who missed birdie putts on the final three holes. Fleetwood and Koepka were playing partners in the final round of the 2017 U.S. Open. Koepka won by four strokes at 16 under, and Fleetwood finished fourth at minus-11.

“He seems to bring it every time it’s a U.S. Open …,” Koepka said of Fleetwood. “He’ll definitely be holding some major championships here soon.”

Koepka was unwavering. He had three birdies and a bogey on the front nine, two birdies and two bogey on the back, and never was rattled. The Florida State alum had what his parents call “that Koepka look” of determination. He’s had that confidence a long time — and they occasionally needed to keep him in check.

“I remember when he made the high school golf team at 12 years old,” Bob Koepka said. “We were driving back and he made the comment, ‘I’m probably going to drop out of school in about four years and turn pro.’ My wife said, ‘You should have seen the look on your face.’

Then, there was the memory of taking his son to the Masters 20 years ago.

“I took him there when he was 8 years old,” Bob Koepka said. “I joked with him then, ‘I got you here. Your job is to get me back.’”

Without question, Koepka has done some heavy lifting.