Brooks Koepka hits out of the bunker on the 16th hole Sunday during the final round of the PGA Championship in Kiawah Island, S.C. Koepka birdied the par-5 hole, but finished two shots behind winner Phil Mickelson. Matt York/Associated Press

KIAWAH ISLAND, S.C. — If Brooks Koepka was rattled by the pressure of trying to win his fifth major championship while getting at best measured support from a rowdy pro-Phil Mickelson gallery, his expressionless face and implacable body language didn’t show it.

The shots he hit told a different story.

Wayward off the tee, uneven with his approach play and unable to rescue himself around the greens, Koepka allowed Mickelson to get away from him Sunday at the PGA Championship.

With a too-late rally on the back nine, Koepka shot 2-over 74 at Kiawah Island to finish at 4-under 284, two shots behind Mickelson. He shared second place with Louis Oosthuizen, a familiar bridesmaid at majors since his runaway British Open victory in 2010. Oosthuizen was runner-up at the PGA for the second time, one of his five second-place finishes in golf’s biggest championships.

Koepka began the day one shot behind Mickelson, in ideal position to match the 50-year-old Hall of Famer with five majors. Instead, Mickelson won his sixth and the 31-year-old Koepka remains tied with Rory McIlroy. Among active players, only Tiger Woods (14) has more.

It was Koepka’s third runner-up finish in a major. His win at the 2019 PGA Championship was between second-place finishes to Woods at the Masters and Gary Woodland at the U.S. Open.

“I’m super disappointed, pretty bummed. 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,” Koepka said. “It’s one of those things where I just never felt comfortable over the putts. I don’t know why, what happened.”

Koepka has finished in the top 10 in 11 of his last 16 majors. But he hobbled his way to a missed cut at this year’s Masters less than a month after surgery on his right knee. He didn’t play again until a week before the PGA, missing another cut.

No wonder 15 of the 16 questions he was asked at his pre-tournament news conference had to do with his health.

But an opening 69 left him three shots off the lead and the story changed quickly. Koepka was in the hunt at another major, focused on outplaying and out-thinking the field as he did in winning back-to-back U.S. Opens (2017-18) and PGAs (2018-19).

He kept himself near the top of the leaderboard with rounds of 71 on Friday and 70 on Saturday. Clearly energized by the wicked difficulty of the Ocean Course, he blamed only himself, not the windy conditions and severe greens, for the poor putts and mental mistakes that prevented him from pulling away.

Koepka grabbed the lead on the first hole Sunday, picking it apart for birdie while Mickelson three-putted for bogey. Though the gallery was unnerved, it was hardly an unexpected start, given that Koepka is nearly two decades younger and came in more than 100 spots higher in the world rankings.

But Koepka gave the lead back just as quickly, making double bogey to Mickelson’s birdie on the par-5 second. On the third, Mickelson made bogey, but Koepka missed a 3-foot putt for birdie that would have put him back ahead.

Koepka never led again.

Although he missed two putts inside 5 feet, it was the driver that got Koepka in the most trouble, and all his misses were in the same direction – left. He missed left on the second, seventh, ninth, 10th, 12th and 13th holes, saving par just twice.

He also put his layup in a waste bunker – yes, to the left – on the par-5 11th, part of an atrocious performance on the par 5s, which he played in 3 over. That dropped him five shots out of the lead.

Koepka picked up three strokes by playing holes 14-17 in 2 under, but the damage was done. Then he had to separate himself from the fans who overwhelmed security guards and swarmed him and Mickelson as they approached the 18th green, taking some shots to his still-recovering knee in the process.

“Yeah, 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,” Koepka said. “Yeah, it’s cool for Phil. But getting dinged a few times isn’t exactly my idea of fun.”

Oosthuizen lurked a few shots behind Mickelson most of the day and could have challenged for the lead on the back nine. But he found the water with his third shot on the par-4 13th and made double bogey.

“Look, 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. My game wasn’t great on the weekend,” Oosthuizen said. “So I just need to work harder on it to get myself in contention again.”