McIlroy finished at 16-under 268 when he won on rain-softened Congressional in the 2011 U.S. Open.
This time it was Koepka's turn to celebrate.
"He's just really, really chill", said Koepka's caddie, Ricky Elliott.
What helped Koepka win the U.S. Open more than anything else was his ability to hit fairways and greens in regulation to stay out of the nasty rough, fescue and bunkers. He finished 10 under.
Emotion? The most he displayed was a light fist pump, his hand clenched a little tighter with each birdie, and a double fist pump on the 18th when he tapped in for par.
"I'm looking forward to my shot at it, I know it's not going to be an easy day, for sure", Fowler said. I put myself in some good chances over the majors over the last few years and never really quite came through. "I just couldn't stand the fact that I'd only won once".
"I was pretty content making pars on the front nine because I knew the kind of day it was", Harman said.
"I kept telling people a year ago after the Ryder Cup", Brandt Snedeker said, "when Brooks figures out how good he is, he's going to be a world beater". "This was just such a solid performance to win a championship that will change his life".
Koepka, who finished at 16-under 272, became the seventh straight first-time victor of a major championship, and it was the first time since 1998-2000 that Americans won their national championship three straight years.
Undeterred, Koepka shot rounds of 69 and 65 to win the qualifier and book his place at Muirfield, where he and Dustin Johnson - the man he would later succeed as US Open champion - played Phil Mickelson and Rickie Fowler in one of the game's most competitive practice round matches. His message was short and sweet.
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"It was a long phone call". "It was like two minutes, it wasn't much". And I'll win if I stay patient and just keep doing what I'm doing.
Some of the loudest cheers at the U.S. Open were reserved for a guy who never really challenged the leaders.
"You've got to tip your cap; he went and won the tournament on the back nine", Harman said. "(He's) starting to figure it out and we're seeing it today.
In 2015 Koepka made $3.5 million playing on Tour, and that's before including any sponsorship deals. Somebody who's got a long career. And I got out there with the putter a little bit (Sunday) and all week. Koepka's only real blemish on the day was a Bogey on the 10th Hole. "I'll try to beat him the next time".
"The way I putted this week was unbelievable too". I think the wrap it was getting for the low scores and everything, I mean it is what it is. "I enjoyed the walk, the golf course is attractive, but I just didn't execute".
The turning point came when Koepka bagged three consecutive birdies, starting with the par-five 14th while Harman carded back-to-back bogeys on the hard 12th and 13th holes.
Harman completed his tournament with an even-par 72, while Matsuyama closed with a 66 to climb the leaderboard.
"There's something about majors where I just focus a lot more", Koepka said. "It's truly an honor".
Koepka signalled his intention from the outset as he birdied his first two holes and the eighth, before a drop shot followed on the 10th.
Koepka said Sunday he had been determined not to let his latest opportunity slip away.
"I hope it does", Stricker said. "It would have been nice".