Standing on the 15th tee Sunday evening at Atlanta Athletic Club, Jason Dufner could see the PGA Championship all but being handed to him. Down below, in the group ahead on the mammoth 259-yard par 3, Keegan Bradley was making a mess — a ball in the rough, a chip in the pond, a triple bogey. When Dufner watched Bradley walk away, he held a five-shot lead over Bradley. He had but four holes to play.

“No lead is safe,” Bradley said.

Really? To that point, Dufner hadn’t so much indicated that he had a pulse. He pulled a 5-wood from his bag, waggled it at his ball — eight times, as is his custom — and swung. Thus began a 100-minute period that not only shook a sleepy major championship right out of its slumber, but produced the summer’s most scintillating golf — whether the contenders were obscure or famous, no-names or stars.

The short version is this: Bradley, a 25-year-old PGA Tour rookie and former all-state skier from Vermont playing the first major of his career, won the PGA Championship in a three-hole playoff over Dufner, a playoff that seemed unimaginable an hour before it started. The long version? Enough folks probably clicked off their televisions that it’s worth retelling, though it might take a while.

“I can’t believe it,” Bradley said.

Nor can most anyone. Start with the abridged version: Bradley followed his disastrous triple with two steely birdies, and he reached the clubhouse after 72 holes of regulation at 8-under-par 272. And Dufner — who had owned the punishing final four holes of the Highlands Course all week — gave the tournament away.

He bogeyed 15 when he hit that 5-wood into the water. He bogeyed 16 when his approach found a greenside bunker. He bogeyed 17 when he three-putted from perhaps 35 feet. And though he forced a playoff with a par at the last, he fell victim to Bradley’s opening birdie in the playoff, his own missed six-footer there, and then another three-putt at 17. He lost by one.

“I’m so new to this situation, as far as trying to win majors,” said Dufner, a 34-year-old journeyman who has never won on the PGA Tour. “I probably don’t appreciate it as much as I might soon.”

So here is Bradley, thin enough that when he turns to the side and sticks out his tongue, he looks like a zipper. He is the nephew of LPGA Hall of Famer Pat Bradley, a graduate of St. John’s — not exactly a golfing powerhouse — and once a winner on the PGA Tour. He chose golf over ski racing as a teenager because, frankly, it was warmer. Now, he is a major champion, the first to win in his first major since Ben Curtis at the 2003 British Open, just the second to do so in 98 years.

“It seems like a dream,” he said. “I’m afraid that I’m going to wake up here in the next five minutes and it’s not going to be real.”

Here, too, is the current state of golf, after the 108th-ranked player in the world outdueled the 80th in a riveting playoff: The past 13 majors have produced 13 different champions. Of those, 10 took a major for the first time.

Where Bradley fits in, it’s hard to say. What is sure: He — not Rickie Fowler, not Dustin Johnson, not Nick Watney, not Bubba Watson — ended the unprecedented run of six straight majors with champions from outside the United States.

For so long, Dufner seemed sure to stop that run. When he made the turn, he had scarcely made a mistake, and held a two-shot lead. He made an I’m-in-complete-control-of-my-emotions birdie at 13 to get to 11 under, and was cruising. And when Bradley muddied up 15 — his tee shot was buried in the thick rough by the green, and his chip never had a chance of holding the putting surface — the tournament seemed over. Bradley walked to the 16th tee down five. He had three holes to play.

“I just kept telling myself, ‘Don’t let that hole define the whole tournament,’ ” Bradley said, adding, “I just didn’t want to be remembered as the guy who tripled that hole and went on to bogey in or something.”

Then, too, came Dufner’s first error, the 5-wood into the water at 15. “I just didn’t hit the shot I needed to hit,” he said. Yet even when Bradley birdied 16, Dufner’s lead was three. For the week, he had made three birdies and six pars on the final three holes.

“Those are tough holes,” he said.

He proved it. After the bogey at 16, Dufner stepped to the 17th tee in time to see the following, across the pond: Bradley, smoothly rolling a 35-footer over the green at the par 3. “It would be a putt that I’ll never forget the rest of my life,” Bradley said. When it dropped, he pumped his fist so hard, his shirt came untucked.

Mentally, by that time, Dufner was untucked. His first putt at 17 ran well past, and he couldn’t hit the comebacker. He fell into a tie, and the wear was showing.

“There’s still an opportunity to win the golf tournament,” he said.

When the playoff began at 16, Dufner regrouped by hitting his approach so well that it nearly grazed the hole before it settled six feet past. Bradley’s answer: an approach that nestled inside Dufner’s ball, perhaps four feet away. Dufner struck his putt tentatively, and he missed, settling for par. Bradley struck his putt confidently, and made birdie.

When Dufner three-putted again at 17, Bradley strode to the 18th tee up two, a lead nearly as commanding as the one Dufner held more than 90 minutes earlier. His caddie told him: Enjoy the walk. Dufner gamely rolled in a 20-footer for birdie, but Bradley’s two-putt par sent his arms in the air, and won him the title.

Back in the 1980s, when Pat Bradley was winning tournaments — 31 of them, including six majors — her mother used to ring a cowbell in her home town of Westford, Mass., not just to celebrate, but to alert the neighbors. That cowbell now sits in the World Golf Hall of Fame. Keegan Bradley’s mother has replaced it with wind chimes. After Sunday, after an improbable comeback, Keegan has other ideas.

“Might have to get that bell out of retirement,” Bradley said. “I’d like to hear it ring once.”