He is all alone on Bethpage Black, the public course he has turned into his private playground.
Koepka wasn't at his best, particularly with his putter on the toughest scoring day of the championship, and he still kept everyone far enough behind to make the final round feel more like a victory lap.
With an even-par 70 that featured a pair of three-putt bogeys, he kept a seven-shot lead and earned another entry in the record book with the largest lead since the PGA Championship switched to stroke play in 1958.
No one has ever lost a seven-shot lead in the final round at any major, or even a PGA Tour event.
That leaves Koepka 18 holes away from joining Tiger Woods as the only back-to-back winners of the PGA in stroke play. He is one round away from becoming the first player to hold back-to-back major title at the same time. Not since Hal Sutton in 1983 has anyone led from start to finish in the PGA Championship.
And a third straight year winning a major? Woods and Phil Mickelson are the only players to have done that over the last 30 years. Tom Watson, Jack Nicklaus and Arnold Palmer are the only others to win majors in three straight years dating to 1960.
Asked if there was any doubt he would win, Koepka said flatly, "No."
He is unflappable in speech and on the golf course. Koepka has never bothered to check his heart rate at rest, but he figures it wouldn't be much different from standing on the first tee of a major championship with a big lead and thousands of rowdy New York fans witnessing a master performance.
"Every time I set up to a golf shot, I feel like I know what the ball is going to do," Koepka said. "And if I don't, then I guess I'd be nervous. ... I'm trying my butt off, and from there, sometimes you need a little bit of luck. But I'd say I'm pretty flat-lined most of the time, as you can tell."
He has all but flattened the strongest field in golf.
Koepka was at 12-under 198, the first time this week he did not set or tie a scoring record.
"I think we're all playing for second," said Luke List, one of four players tied for second.
Dustin Johnson tried to make a run with six birdies, only to stall with five bogeys in his round of 69. No bogey was more damaging than the 18th. A drive into the fairway would have given the world's No. 1 player a reasonable shot at birdie. Instead, he sent it right into bunker, came up well short into the native grass, left the next one in the bunker and had to scramble to limit the damage.
That kept Johnson from joining his close friend in the final group.
Koepka will play the final round with Harold Varner III, whose week began with plans to play a practice round with Woods on the eve of the PGA Championship until Woods called in sick. Varner birdied the 18th to cap off a bogey-free 67 and lead the group at 5-under 205 that includes Jazz Janewattananond (67) and List, who holed two shots from off the green for a 69.
Jordan Spieth did not put any pressure on Koepka at all. Playing in the final group on the weekend for the first time since the British Open last summer, Spieth didn't have a realistic birdie chance until the sixth hole, and he missed that one from 8 feet. He shot 72 and was nine shots behind.
Spieth would not speak to a reporter after the round.
There was simply no stopping Koepka, who is one round away from a fourth major in his last eight tries and a return to No. 1 in the world.
The plan for Sunday was no different from the previous three rounds.
"It doesn't really matter. I'm just trying to play good golf," Koepka said. "If I can get off to a good start tomorrow, these first six holes are very scorable. I feel like if you can get 1 or 2 under after six, you're in a good spot."
That's what worked on Saturday.
Koepka had birdie chances on the opening six holes and converted two of them, from 5 feet on a blind shot up the hill at No. 2, and a gap wedge that landed next to the pin and settled just over 2 feet away on No. 5.
His only struggle was missing a 2-foot par putt on the ninth hole for a three-putt bogey, and then missing the 10th fairway to the right to set up another bogey. The most important putt for Koepka was just under 5 feet for par on the 11th, which kept him from three straight bogeys.
And then he was back in his groove.
List ran off three straight birdies, chipping in from 70 feet on No. 12, holing a 30-foot putt on the par-5 13th and making a 15-foot putt on the 14th. That pulled him within five, but it wasn't long before Koepka birdied the 13th and List began missing enough shots that it finally cost him.
Johnson has the most experience and skill among those chasing Koepka, if he even allows there to be a chase.
"It's going to take something special to catch Brooks, but it's doable," Johnson said. He then tried to work out the math, and then he stuck to a more practical outlook.
"I'm going to need some help from him," Johnson said. "And then I'm going to have to play very, very well."
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