Unlikely buddies, the kid from Florida State and the superstar from Northern Ireland have formed a friendship based on age, golf and residences in the Palm Beach area of Florida.
It doesn't hurt that they are impressive talents, bound my mutual respect and perhaps a bit of friendly competitiveness.
McIlroy, of course, is golf's reigning world-beater, a solid No. 1 who captured the Omega Dubai Desert Classic for his 10th European Tour title and fourth victory in his last 12 starts.
Koepka does not possess the same level of prominence, taking a more obscure and yet adventurous route to golf's big stage, but his victory Sunday at the Waste Management Phoenix Open was no less impressive.
Boosted by an eagle at the par-5 15th hole, Koepka parred the last three holes at TPC Scottsdale to secure his first PGA Tour title and move into the top 30 in the Official World Golf Rankings.
The win was Koepka's second victory in four starts, the last coming in November at the Turkish Airlines Open on the European Tour. He's got a ways to go to get in McIlroy's league, but he can share bragging rights on this day.
"It's cool," said Koepka of his friendship with McIlroy. "We're the same age pretty much. We've got common interests. Both like sports, hanging out, doing whatever.
"I don't want to say he's been somebody you look up to a little bit, but he's an unbelievable player. He's got experience, too. He's been a professional since he was 18. So he's got a little more under his belt than I have, so we can relate in that aspect. But he's got the experience.
"So I kind of pick his brain a little bit and watch him, how he goes about things and how fearless he is. The driver ... he just pulls driver every hole. Just trying to learn a little bit just from watching him play. It's been a big help."
Koepka, 24, who was a two-time Atlantic Coast Conference Player of the Year at Florida State, turned pro in 2012 but failed to earn his PGA Tour card at the annual Qualifying Tournament, which then gave immediate entry to the PGA Tour.
It turned out to be the best thing that could have happened as it forced him overseas.
Koepka had status on the Challenge Tour -- the European Tour equivalent to the Web.com Tour -- and won tournaments in Italy, Spain and Scotland in 2013 to earn a promotion to the big tour as well as a full exemption in 2014.
Along the way, he played in countries such as England, France, Belgium, the Czech Republic, South Africa, Kenya and Portugal. He estimates having played tournament golf in more than 20 different countries.
And he impressed by doing so. After earning his third Challenge Tour victory in June 2013 in Scotland, he boarded a late flight to London so he could participate in the next day's 36-hole Open Championship qualifier. With little sleep, Koepka earned a spot, secured a practice round with Phil Mickelson at Muirfield, and earned the eventual champion's praise.
"I can see why he earned the right to get on the European Tour by winning three times so quickly on the Challenge Tour," Mickelson said.
He also showed that there is another way to climb into golf's top level outside of playing the Web.com Tour. Like his roommate Peter Uihlein, who is exempt on the European Tour and tied for 13th in Dubai, Koepka is a full-time member of the European Tour and plans to keep it by playing in the required number of events.
He will be aided by being exempt for the major championships, which count on both tours, as well as the World Golf Championship events. He had already qualified for the Masters by tying for fourth at the U.S. Open last summer.
"Going that route, going over to Europe, it toughened me," Koepka said. "It was a blessing in disguise. At the time I was extremely disappointed, as I'm sure Jordan Spieth was, too. Because I think we were at the same spot, finished two out of [exempt status].
"But that just toughens you up, your drive. It makes you want to get out here that much more."
He's here, and Sunday's victory assured that it won't be a short stay.