Brooks Koepka's fitness was a topic of discussion prior to the US PGA Championship and while he secured a share of the runners-up cheque, the four-time major winner was "super disappointed" with his performance.

Koepka went head-to-head with Phil Mickelson, who came out on top by two strokes in a stunning display that saw the American veteran become the oldest major champion in golf history on Sunday.

A two-time PGA Championship winner, Koepka signed for a two-over-par 74 as he was unable to capitalise on Mickelson's final-round 73 in South Carolina.

Koepka's short game was his downfall – the 31-year-old ended with a double-bogey, four bogeys and four birdies to finish second alongside Louis Oosthuizen at Kiawah Island.

American star Koepka has been plagued by injuries since winning back-to-back PGA Championships in 2019 and a fourth major title in three years, undergoing knee surgery in March before missing the cut at last month's Masters but his title tilt did not mask his frustration.

"Just how bad I putted the last two days," Koepka said when asked what part of the result was hard to stomach. "Three days, actually. It felt like tap-ins I was missing. Never felt comfortable, and you're not going to win if you do that.

"The thing was, Phil played great. That whole stretch when we turned after four and five and played those holes, it's into off the left for me and that's quite difficult for a right-handed player. And it suited Phil right down to the ground, and I thought he played that entire stretch from about six to 13 so well. So you know, I'm happy for him... It's pretty cool to see, and you know, but a bit disappointed in myself."

"I'm super disappointed, pretty bummed," Koepka added. "I'm not happy. I don't know if there's a right word I can say on here without getting fined, but it hurts a little bit. It's one of those things where I just never felt comfortable over the putts. I don't know why, what happened.

"I spent all weekend, the weekend before working on it and it was great, and you know, just over did it. I was trying to get my hands a little lower and ended up getting my hands too far low one under and actually ended up getting further away from the ball. The last nine, I just tried to go back to what I've always done and I felt like I was hitting better putts. I just wish I would have done it sooner."

Oosthuizen – winner of the 2010 Open Championship – carded a one-over-par 73 to earn a share of second spot.

The South African recorded his best major performance since finishing tied for second at the PGA Championship in 2017.

"I feel like I'm playing my heart out to get a second major, and I do know I have the game to do it. This was close," said Oosthuizen.

"My game wasn't great on the weekend. It was better today than yesterday. So I just need to work harder on it to get myself in contention again."