So was Tiger Woods, which apparently created a different environment at Bellerive Country Club.
Woods shot a 64 to finish the tournament.
That left Woods five strokes off the pace of United States compatriot Koepka, who birdied five of his first nine holes to make the turn on 13-under par, three shots ahead of Americans Rickie Fowler and Gary Woodland with Australian Adam Scott, the 2013 Masters champion, and American Kevin Kisner another stroke adrift.
Just look at this video of Woods arriving at Bellerive for the final round. Instead of being near or at the top of the leaderboard, he finds himself four shots back of Brooks Koepka, who leads the field by two.
Those errors brought Woods back into contention. The others are Woods, Jack Nicklaus, Ben Hogan and Gene Sarazen.
"The difference between the level I think I've played at and the top level is very fine and really after a little while it's just that little bit of confidence that you lack", Scott said after his six-birdie round was blemished only by a closing bogey on the ninth. "I hit the shot how I wanted to and just came up short".
Woods, with a 2:35 p.m.
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"I can live with that", he said Friday after a 4-under 66. (Next year it will move to the second major, after The Masters, and will be played May 16-19 at Bethpage Black). Kopeka birdied the first two holes to take the lead. He drained his longest putt of the tournament, a 19-footer putt for birdie, and pumped his fist to celebrate. "I bogeyed the last, but sometimes when you have bad shots, you're going to make bogey".
After wasting one chance to put it away, Koepka ran kept attacking flags and ran in birdie putts of 10 feet on number 15 and 7 feet on number 16 to end the drama. That's just the nature of the way it's playing right now.
He missed. And then he missed the next from 4 feet for birdie and ended his day with 10 straight pars. "But the nice thing about the delay and going back out tomorrow morning, we'll get fresh greens".
Woods hit zero fairways on the front nine but was able to repeatedly pull off miraculous recovery shots, the same ones that made him so great in his glory days.
Three-time major victor Jordan Spieth, needing only the PGA to complete a career Grand Slam, opened birdie-birdie but took a double bogey at the par-3 sixth then followed that with three consecutive birdies.
It left a sense of anti-climax, but Woods is still within reach of his 15th major, a situation few would have predicted at the start of a year which has seen the 42-year-old resurrect his career after debilitating injuries.