Koepka took a tournament-record seven-shot lead into the final round at Bethpage and was still six clear with eight to play, only to bogey four holes in succession and see Dustin Johnson close to within a single shot.
By this stage, the raucous New York crowd were firmly on Johnson’s side and were chanting “DJ, DJ” as Koepka bogeyed the 14th, but Johnson would drop shots on the 16th and 17th to leave Koepka to seal a hard-fought victory.
“It’s New York. What do you expect when you’re half-choking it away?” Koepka said with a smile.
“I think I kind of deserved it. You’re going to rattle off four (bogeys) in a row and it looks like you’re going to lose it... I’ve been to sporting events in New York. I know how it goes.
“I think it actually helped. It was at a perfect time because I was just thinking, OK, all right. I’ve got everybody against me. Let’s go.”
Koepka steadied the ship with vital pars on the tough 15th and 16th holes and eventually signed for a closing 74 - the highest final round by a major winner in 15 years - and a two-shot victory.
The 29-year-old replaced Johnson as world number one on Monday and with the US PGA moving from August to May this year, he is the first player to hold back-to-back titles in two majors at the same time following his US Open wins in 2017 and 2018.
“Phenomenal. I think that’s a good word,” Koepka said when asked to sum up his recent form in major championships.
“Yeah, it’s been a hell of a run. It’s been fun. I’m trying not to let it stop.
“It’s super enjoyable and I’ll just try to ride that momentum going into Pebble (Beach, venue for the US Open). I mean four of eight (majors), I like the way that sounds.”