The muscular American, who started the day one back of the lead, tamed his nerves and a wind-whipped Erin Hills carding six birdies, including two to kick-start his round, against a single bogey for a five-under 67 to finish four clear of compatriot Brian Harman (72) and Japan's Hideki Matsuyama (66).
Koepka has been tipped for the top since winning three tournaments on the Challenge Tour in 2013 to earn instant promotion to the European Tour, but the third win was in serious danger of never happening.
"I needed to make that if I was going to win this tournament", he said.
"On the Sunday I did not play how I wanted to and it was windy and the conditions were tough. That's what I kept doing", he told a news conference. I don't want to say I was homesick, I was just exhausted of golf, exhausted of travelling. "I get a lot of experience in the last couple of days", Li added.
"I felt confident all week and to feel as confident as I did on a Sunday of a Major and coming down the stretch was great", he said. I don't know what it is. "I think I had played so many weeks in a row, not a day off, it really got to me".
However, after his auto suffered a tyre blowout near Edinburgh, he was forced to change plans, get a few hours sleep in a hotel and catch a 6:30am flight to Heathrow, arriving at Sunningdale shortly before his tee time.
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Rickie Fowler's hopes of ending his wait for a major faded with an error-strewn performance, a level-par 72 leaving him six shots adrift.
Johnson, who didn't make the cut, called Koepka on Saturday night. "Reminded him about that a couple of times since". Koepka won the Turkish Airlines Open in 2014 over Ian Poulter and Henrik Stenson.
Koepka's victory, and before him Sergio Garcia at the U.S. Masters, show the folly of jumping on the bandwagon of whoever happens to be hot on any given week, or month.
He won that tournament, made it to the regular European Tour, and then made it home to the PGA Tour. "And I felt like I put myself in contention so many times", Koepka said. I was just exhausted of golf. "It's one of those things, not a big fan of losing". Between conversations I've had with Claude [Harmon III] and Pete Cowan sat me down, I think, on Tuesday, and kind of had a conversation with me just kind of staying patient, not trying to win.
NOTEWORTHY: Koepka is the seventh straight first-time major victor.
The celebration was about as muted as you'll ever see from someone who just won his first major.
His confidence in the final round came, in part, from a short conversation the night before with Dustin Johnson, a friend who had been through his share of close calls and heartbreaks before finally becoming a major champion at the U.S. Open past year.
Ranked 22nd in the world, Koepka shot 2-under or better in every round of his fifth US Open.