The Englishman could only watch as Koepka finished with a flourish to take home his maiden Major title with a winning score of 16-under par.
Fleetwood shot a level-par round of 72 to finish fourth with -11.
And he has conceded Koepka’s performance was 'unbelievable'.
“You never know how you're going to react being up there in the final round - you've got to deal with that and it's all new for me,” Fleetwood said.
"I felt really comfortable yesterday - I didn't play how I wanted to today, it was fine still but you can't do too much about [how Koepka played], can you?
"He was brilliant, the shots he hit down the stretch, you can't describe how hard some of them are.
"He was phenomenal - I would've like to have played like that. I played well enough to do it yesterday but didn't quite have it today.
"It was windy and he shot five under, fair play."