Brooks Koepka showed he's human on Sunday, the normally unflappable American struggling through the final round of the PGA Championship and falling short of a bid for a historic three-peat.

The two-time defending champion was trying to play his way to a third consecutive title but ended up making six bogeys en route to a four-over 74.

He finished four rounds at three-under 277, 10 strokes back of winner Collin Morikawa, who picked up his first career major title.

"To make the turn at four-over was disappointing, to say the least," said Koepka. "It's my first bad round in a while in a major. 

"Hey, it wasn't meant to be. Three in a row, you're not really supposed to do two in a row looking at history. I got two more the rest of the season and we'll figure it out from there."

When it comes to major championships, Koepka has been the man to beat. Dating to the summer of 2015, he has been the game's most dominant force on its biggest stages.

Koepka has won majors four times, two US Opens and two PGAs. He defended his PGA crown a year ago when he edged Dustin Johnson at Bethpage Black in New York.

Sunday's final round was expected by many to be a showdown between third-round leader Johnson and Koepka.

Koepka poured fuel on the fire Saturday night by taking a verbal jab at Johnson after finishing his third round. 

"I mean, I like my chances. When I've been in this position before, I've capitalized. I don't know, he's only won one (major). I'm playing good," Koepka said.

But winning majors is about hitting fairways and greens and Koepka was unable to do either with any consistency on Sunday.

He started his final round with a par before making bogey on the second hole. The wheels started to fall off at the par-4 seventh, where he made the first of three consecutive bogeys. He had two more bogeys late in his back nine.

"Every time I hit it in the rough today I got probably the worst lie I've had all week," Koepka said. "If you're going to put it in the rough out here, it's pretty tough.

"The green speeds this weekend I never really got down. I felt like the putting green was a little bit quicker.

"And I just never quite got putts to the hole to make anything. So you don't do that, it's going to be tough."