The PGA of America have apologised to both Phil Mickelson and Brooks Koepka, after the at-times wild crowd scenes in the latter stages of their final round on Sunday.

Mickelson became the oldest major winner in history on Sunday, winning his sixth major title when he defeated Koepka and Louis Oosthuizen at Kiawah Island.

However, with 10,000 fans in attendance for the final round, crowd control became a major issue as the players made their way up 18, a situation Mickelson described as, "slightly unnerving, but exceptionally awesome".

While Mickelson was able to enjoy the scenes, his playing partner Brooks Koepka didn't feel so comfortable, saying he was all too wary about his recent knee surgery.

"It would have been cool if I didn't have a knee injury and got dinged a few times in the knee in that crowd because no one really gave a shit, personally," he said.

"But if I was fine, yeah, it would have been cool. Yeah, it's cool for Phil. But getting dinged a few times isn't exactly my idea of fun.

"I don't think anybody really understands until you actually you're coming out of surgery how... I mean, even when I was doing rehab and there's five people kind of standing by your knee, you get a little skittish."

PGA of America CEO Seth Waugh has since confirmed they have apologised to both players for the wild scenes, saying the security personnel had been "overwhelmed" by the size and enthusiasm of the crowd.

"While we welcome enthusiastic fan engagement, we regret that a moment of high elation and pent-up emotion by spectators on the 18th hole during the conclusion of yesterday's historic PGA Championship briefly overwhelmed security and made two players and their caddies feel vulnerable," he said.

"We always put player safety at the top of our list and are grateful that order was restored. I have spoken to both players and apologized on behalf of the Association," he added.