Rory McIlroy leads a group of young golfers who lead the field entering Sunday’s final round at the PGA Championship. (Brian Spurlock/Usa Today Sports)

There was a then-and-now feeling to the PGA Championship at Valhalla. A generational duel has been forming for some time in golf, but it never seemed so stark as it did Saturday, with a 19-year age difference between Phil Mickelson and those retina-blinding boys Rory McIlroy and Rickie Fowler, in their pulsing colors. It used to be the kids chasing the older guys. But now it was the older guys chasing the kids, and all of sudden, Mickelson looked like he had spent too long a time in the sun.

The third round at Valhalla felt breathless and fast — the 25-year-old McIlroy swung fast, he walked fast and he scored birdies fast, with fire-colored clubs and bulging sleeves in explosive blue, en route to a 67 to stand alone at 13 under par. By comparison, the 44-year-old Mickelson seemed to wear twilight on his shoulders in his greys and whites as he tried to keep pace, fighting his way to 10 under. By the end of the day, a great tournament also seemed to be something more, a hinge in time.

The game’s formerly dominant player, 38-year-old Tiger Woods, seemed to admit as much after he had missed the cut Friday with a bad back. Asked whether this tournament had made him feel old, he said: “I felt old a long time ago. It’s darn near 20 years out here.”

No matter who wins Sunday, this much is certain: McIlroy and the others in his age group are no longer fighting their baby fat. They are fully matured players who seem to say, “Get out of the way” with every stroke. Of the five players atop the leader board bunched within three strokes of each other, four of them were 28 or younger. There was the 28-year-old Austrian Bernd Wiesberger, a stroke behind McIlroy after shooting 65; the 25-year-old Fowler at 11 under with his 67; and 26-year-old Jason Day, yet another stroke back after a 69, tied with Mickelson, the only outlier in age.

McIlroy, Fowler and Day are just 18 months apart, and they showed a combination of recklessness and style in those shirts that looked like colors that glow from laptops. But they also showed resilience, a springiness, an ability to snap back from bad shots quickly. Take Day’s remarkable recovery on the second hole, a 500-yard par-4 where he lashed his drive so far offline it flew over a creek and into some long bluegrass, where announcer David Feherty found it for him. Day kicked off his shoes and rolled up his pants, waded across the creek, took up a stance in his bare feet and whipped a beautiful recovery shot back onto the golf course and went on to save par.

There were other accomplished players out there, of course, tweeners like Adam Scott (34) and Louis Oosthuizen (31). But the low scores undeniably came more easily to the young heat-seekers, who have fewer burdens, concerns, interests and responsibilities; who don’t have families and children like Mickelson and Woods; and who can afford to eat, sleep and breathe the game. “What else do I have to do?” McIlroy asked earlier in the week. “I get up in the morning. I go to the golf course. I go to the gym. It’s just sort of — it’s just my life at the minute, you know?”

They swung freely, with no aches — and no sense that anything could ever hurt. They had no concept of how it might feel to have psoriatic arthritis, like Mickelson, or disc problems, like Woods, who was so stiff “I couldn’t make a back swing,” he said. “I can’t get the club back.” They had no sense of what the game could do to them, how it could gradually wear away at even the strongest body over the years and literally diminish it.

“I was 6 feet tall, and now I’m 5-8,” Jack Nicklaus observed to ESPN radio earlier this week. “I lost four inches because I have no discs. I wore them out playing golf.”

They exuded limitless potential and ran off birdies in clusters. Coming into the PGA Fowler had the lowest aggregate score under par of any player in the majors this year, a scorching 18 under, between his tie for fifth in the Masters and ties for second in the U.S. and British Opens. To which he has now added scores of 69-66-67 at par-71 Valhalla.

As for McIlroy, he is now a total of 30 under par in his past seven rounds of majors counting his 67 on Saturday, which concluded with birdies on three of his last four holes after struggling to hold on to a piece of the lead for much of the day against onslaughts from Fowler, Day and Wiesberger. And which came with an ease that was almost unsettling, that at times seemed to defy the basic physics of the game. Take his birdie on the 508-yard par-4 16th, which played as the third-hardest hole on the course for the rest of the field. He drove it 336-yards. That left him 171 yards away from the pin — and all he needed to get there was a high 9-iron, which drifted gently to 18 inches from the cup.

“It’s not the biggest lead I’ve ever had, but I’m still in control of this golf tournament, and it’s a great position to be in going into tomorrow,” McIlroy said, exhibiting no sense of pressure. “Loving it. I’m loving it. It’s where I want to be.”

It’s an easy game — when you’re that young.

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