Out in 41, he restored some semblance of order with a two-under-par back nine.
By the time the final groups reached the turn, the championship became a three-man shootout.
Koepka broke away from a tight pack with three straight birdies on the back nine Sunday at Erin Hills and closed with a 5-under 67 to win the U.S. Open for his first major championship.
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"On 10 I just hit a poor putt, not enough speed, and 11 the same thing", he said, underlining the importance the putt on 13. Just the qualifier itself was a huge thing for me just that I made it was a relief that I wanted to be here so bad. "Unfortunately, that's really all I was dealt, so it's what I had to try to do".
Matsuyama's best round of the day included eight birdies offset by two bogeys. 14-16 to create plenty of distance between himself and the rest of the field.
Tied with Koepka at 13 under, Harman missed his par putt at the par-4 12th to fall a shot off the lead.
"Brooks played great, he went out and won it, so I gave it my best, just came up a little short", Harmon said.
To hear Koepka on Sunday evening, the gleaming silver U.S. Open trophy next to him on the dais, his days of underachieving are over.
Koepka's winning total of 16-under equals the lowest score to par for a U.S. Open, matching Rory McIlroy's victory at Congressional in 2011.
Players griping about the course is a staple at U.S. Opens and, tellingly, there was little of that once the tournament began Thursday.
"It feels fantastic to get my name on this trophy with so many other great names", Koepka said after becoming the seventh first-time major victor in succession, a run stretching back to Jason Day's victory in the 2015 US PGA Championship. Below Harman on 12 under, three players are exhausted on 11 under - Thomas, Brooks Koepka and England's Tommy Fleetwood. Tommy Fleetwood, Patrick Reed, and Si Woo Kim only managed even-par or worse.
He finished at 16-under for the tournament, tying the lowest 72-hole score relation to par in the tournament's history. On Sunday, he picked up his sixth top-10 finish and third top-3 finish since joining the PGA Tour in 2012.
But it made me feel a lot better seeing Brooks shoot 5-under.
Right now, there may not be any sport in America where you can be as good as Koepka - nearly everyone in golf thinks that the ex-Florida State star, ranked No. 22 in the world, is a major figure of the game's future - and yet have so many genial, obsessed, talented players of your generation, your own buddies, who are constantly trying to grind you to dust by their sheer numbers.
A changing of golf's old guard has been underway for some time but the 117th US Open signaled that it can be considered complete with neither Phil Mickelson and/or Tiger Woods teeing up at major for the first time since the 1994 US Masters.
Harman dropped back on the back nine after going bogey-free on the final nine holes over his first three rounds.
It was only fitting that Koepka left Erin Hills with yet another record matched or broken.
"It was kind of bombs away", he said. But he also hit 88% of fairways and 86% of greens.
FSU head coach Trey Jones, who coached Koepka, took part in an in-game interview at the College World Series on ESPN2, talking in the middle of the fourth in a game between Florida and TCU.
In round two Casey had rallied to a one-under 71 despite a triple bogey, but when another triple came the next day he wasn't able to recover and slipped down the leaderboard with a three-over 75 to four over.
That winning feeling was so sweet for Brooks Koepka after he sunk the final putt on the 18th.
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