KIAWAH ISLAND, S.C. — For a while, it looked like a Saturday somebody ought to paint. Phil Mickelson, 25 days shy of 51, made the turn, blasted one 310 yards into the fairway at No. 10, knocked one to seven feet, made the putt for a birdie and led by five holy-mercy strokes over a field chockablock with whippersnappers. Roars ricocheted around the green, the truth impossible, the Ocean Course like some dreamscape.

Mickelson had played his previous 18 holes at 10 under par, stretching back into Friday, and as he threatened to run off with the 103rd PGA Championship, he walked with serial thumbs-ups toward spectators who long since knew his name by heart. What in the world was happening here?

Reality, that habitual bummer, made its tardy arrival, of course. Once Mickelson had missed a seven-foot chance at a birdie on No. 11, then ventured into a ticklish side edge of a bunker on No. 12 toward a bogey, then whacked into the water on No. 13 toward a double bogey, the second men’s major tournament of the year had itself something else sumptuous.

It had itself a potential duel.

Just as Mickelson started looking very much like the oldest major winner ever, here came an incredible athlete at 31, Brooks Koepka, the very kind of person who could achieve contention after a spring with knee-surgery rehab and two missed cuts. He birdied No. 16 after darn-near eagling the thing. Soon they stood tied at 7 under. By Saturday night, Mickelson led Koepka by one after Koepka bogeyed No. 18 to slide to 6 under, and Sunday had promise.

That promise differed from hours before but was promise nonetheless, with 2010 British Open champion and 2012 Masters playoff runner-up Louis Oosthuizen at 5 under, Kevin Streelman at 4 under and Christiaan Bezuidenhout and Branden Grace at 3 under. With Oosthuizen, Bezuidenhout and Grace, it meant a top six fully half-South African. Yet you can peg the two Americans at the top as the attraction without even sounding jingoistic.

They’re some duo. Mickelson was 43 when he stormed to the 2013 British Open’s claret jug with one of the great rounds of the sport’s history, a closing 66, earning him a fifth major title. Yet if Koepka were to use his massive major moxie to surpass Mickelson on Sunday, he, too, would reach five major titles, all bunched within a 47-month period.

“I think there’s going to be a lot of people following that group,” said Oosthuizen, who will have fewer distractions from the penultimate group — not that he hasn’t long since showed a knack for handling a lot.

“I left a lot out there,” said Koepka, who walks the courses of life with a calm and full knowledge of his soaring capabilities. He also said: “That was the worst putting performance I think I’ve ever had in my career. Can’t get much worse. I thought 70 was about the highest I could have shot today.”

He shot a 70, and he closed with a seven-foot putt for par that seemed to groan as it missed. He still had forged the calmly remarkable yet again. He had bolted into the final pairing of a major after surgery March 16 for two things that sound shy of nice — kneecap dislocation and ligament repair — and he had snared a share of the story.

The story did start out one-starred. Around 2:30 p.m., out walked Mickelson, a vision in all black, sunglasses reflecting outward as ever. “Phil! Let’s go, Phil!” people shouted. He flung down about five balls on a practice green and began a few chips.

He headed to the No. 1 tee, and people bunched around, if only four-deep given the pandemic crowd limitation of 10,000. His career has known such expanse that the starter announced his PGA title from an event held 16 years ago, at Baltusrol in New Jersey, and that event had come well into his career.

He drove it, and somebody yelled, “Bomb!”

“Let’s go, Phil!” went an ear-mauling scream.

Off he went, and soon he started doing artful things with his clubs. When he knocked it out of the sand on No. 3 from 99 yards, even the sand itself seemed to fly upward with dramatic flair, and the ball flew beautifully to the right of the cup, then slid gently left to within two feet.

That became his second straight birdie. He had another on No. 6, from 17 feet. He had another on the par-5 No. 7 after reaching that green in two, and when he had trouble on No. 9, needing to get up-and-down from a bunker, he had a rake in his hand, did some raking, addressed the ball and produced something near-magical, an obedient beauty that rolled still at four feet. By the end of No. 10, he had visited a good bit of sand while avoiding the misadventures that have helped backlight his career.

“I felt I had a very clear picture of every shot,” he said, “and I’ve been swinging the club well, and so I was executing.”

In the quieter parts of the course, Koepka noticed but didn’t swoon. He long has simplified his majors and has finished in the top five in an astounding 10 of 27. “Yeah, I saw Phil was at 10 and I was at 5, but just go about my business,” the steady Floridian said. “I can’t control what he’s doing. I just need to play better. Simple.”

Soon No. 12 found Mickelson awkwardly straddling a bunker, the fans lining the opposite side of the fairway noting the gymnastics of his posture. No. 13 found him following Oosthuizen smack into the water, making two tee shots after the first dunked into the middle of a creek. “So, yeah, it’s just an example of losing the feel and the picture of the shot, and I get a little bit jumpy, a little bit fast from the top, and it just — when that happens, I get narrow and I end up flipping it,” he said.

As he got narrow, so did the margin over a contender who knows how to contend. As a bonus, Koepka hasn’t even had the occasional pain pushing off that right knee on drives. “I have not actually,” he said. “I have not had that once this week. Very pleased. I think it’s even — the strength has increased.”

A story of one became a story of two, with more possibilities just behind, and it magnified what Jordan Spieth said much earlier when he finished getting to even par with plans to go back to his digs and watch Mickelson. “It’s theater,” he said, and now it looks like a whole lot of it, maybe even too much for just one painting.

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