But when Koepka knocked his approach shot snug on 16 to push the lead to even par with two holes to play, there was no question that he was going clear every hurdle Shinnecock Hills provided this week.
Fleetwood must console himself with a cheque for $1.2m and a share of history.
Players who travel widely become more accomplished and well rounded golfers; after all, a strong wind in one country can be very different to that in another.
But Koepka steadied himself and ground his way home in the traditional style of a U.S. Open victor, making par on six of the last seven holes and birdieing the par-5 15th to give himself some breathing room.
Fleetwood took full advantage with a stunning final round of 63, agonisingly missing from eight feet for birdie on the 18th to equal the all-time major record of 62 set by Branden Grace in last year's Open Championship at Royal Birkdale.
"I don't want to say I didn't think I could do it, but I knew that it was going to be that much more hard", Koepka said after closing with a 2-under 68 and a 1-over 281 total.
Fleetwood, playing more than two hours ahead of the overnight leaders and starting the day six shots off the pace, had threatened to steal a stunning victory after reeling off four consecutive birdies from the 12th hole.
"I didn't miss it until I knew I wasn't going to be at Augusta", he said.
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He recovered with three fantastic rounds, however, and the putts the he holed down the stretch on the final day to retain his title by two shots showed great courage.
"I'd say it's a little bit over the line", Henrik Stenson said after shooting a 74 despite being "pretty happy with the way I played today".
Finau spoke just as Johnson, with a halfway total of four-under 136, teed off with a four-stroke lead over Piercy and Charley Hoffman, while a high-calibre group of five was another shot further back at one-over.
"I made an incredible four", Koepka said.
Koepka became the first man to turn that impressive trick in almost three decades by handling a Shinnecock Hills course that varied from beastly to benign. This afforded viewers with a much closer U.S. Open than many imagined after Dustin Johnson appeared to be in control on Friday. "I was hitting the ball well and I was putting the ball well, but I just had to keep going". Johnson's Saturday brought him back to the field and during the final round it turned into anyone's game.
There have been exceptional golfers who have spent their lives unsuccessfully chasing big titles, however Koepka seems to have a gift for these big moments, often walking away with success.
"I knew what score it was for".
Everything about 2018's event from the scoring perspective was the inverse of the 2017 U.S. Open at Erin Hills.
A hot putter kept him in front as he made clutch pars at the next three holes. There definitely will be one more major at least.