AP photo by David J. Phillip / Fans cheer for Phil Mickelson on the 16th hole of the Ocean Course during the third round of the PGA Championship on Saturday in Kiawah Island, S.C.

KIAWAH ISLAND, S.C. — Phil Mickelson could have done without the thrills Saturday in the PGA Championship.

Losing a five-shot lead in a span of five holes on the back nine of the Ocean Course. One tee shot going into the water. Another that ended up under a cart tire. It was all part of a wildly entertaining day at Kiawah Island that ended with Mickelson nearly holing a flop shot that can test the nerves of just about any 50-year-old but him.

That brought Mickelson to the cusp of a historic moment not many could have seen coming.

When he curled in a four-foot putt on the 18th hole for a par save and a 2-under 70, it gave him a one-shot lead over Brooks Koepka and left him 18 holes away from becoming the oldest champion in the 161 years of golf majors. Mickelson is the oldest player with a 54-hole lead in a major since Tom Watson at age 59 in the 2009 British Open.

That didn't end well for Watson, who lost a playoff to Stewart Cink, but the opportunity is all in front of Mickelson. Julius Boros was 48 when he won the 1968 PGA Championship, and that record is under threat.

Asked to describe such an occasion, Mickelson was too busy glancing at his watch. Sunlight was fading, and he wanted to practice. These opportunities don't come along as often as they once did.

"I'm not really dwelling back on what took place today," he said.

The final round should get his attention, starting with the guy who joins Lefty in the final group. Koepka survived what he called the worst putting of his career. Statistically, he was middle of the pack, but he missed a six-footer for par on the final hole and made a bogey for a 70 that cost him a share of the lead.

No matter. At stake for Koepka is a shot at his third Wanamaker Trophy in four years. No one has won the PGA Championship so often so quickly since it switched to stroke play in 1958.

"I'm in the final group," said Koepka, also a two-time U.S. Open champion. "That's what you want."

Mickelson was at 7-under 209. For all his success in the majors — five victories, runner-up finishes in all four of them — this is only the third time he has held a 54-hole lead in one of the big events.

Koepka, shaking off effects from ligament surgery on his right knee that limited him to two tournaments in three months before arriving at Kiawah Island, wasn't surprised to have another shot at a major. The 31-year-old from Florida already has four of them in the past five years.

"It just feels good, feels normal. It's what you're supposed to do, what you practice for," he said. "I'm right where I want to be, and we'll see how tomorrow goes."

South Africa's Louis Oosthuizen started the third round tied with Mickelson and had a long three-putt bogey. The 2010 British Open winner never caught up, though he had his chances until missing a four-foot birdie putt on the par-5 16th and a five-foot par putt on the par-3 17th. He wound up with a 72 and was two shots behind.

"Probably the worst I've played in a while," Oosthuizen said. "I was just sort of fighting to stay in it, and at the end there started judging the greens wrong, and everything just fell apart. All in all, two behind going into Sunday, I've got to take a lot of positives out of that."

At least they have a chance.

Mickelson broke away quickly with four birdies in seven holes, and he even managed to avoid losing his focus, something he has admitted has become a challenge lately. One distraction came from the fourth fairway, when Mickelson saw a drone in the air left of the green and said to a CBS spotter, "Can you radio to the TV guys to get the drone out of the flight of my shot?" He saved par from a back bunker.

He went out in 32 — Mickelson played the front nine on Friday in 31 — and was five shots clear when he walked off the 10th green. Five holes later, the lead was gone.

Mickelson badly missed a seven-foot birdie attempt on No. 11. He pulled his tee shot into a bunker on No. 12 and had to play back to the fairway, leading to his first bogey of the round.

And then he drove into the water on the 13th with his 2-wood, had to hit his third shot from the tee because of where he thought it crossed the hazard line, and missed a 12-footer to take double bogey. Ahead of him, Koepka rattled off three birdies.

Mickelson, without so much as a top-20 finish on the PGA Tour in the last 10 months — he did win his first two starts as a golf senior on the PGA Tour Champions — is going after his first major since the 2013 British Open, and the final hour made it clear it might not be easy.

It will be loud, though. The gallery is the largest at a major since the pandemic began — the PGA of America has said there would be 10,000 people, a number that felt far greater — and Mickelson was the object of their raucous shouting.

Kevin Streelman bogeyed the 18th for a 70 and was alone in fourth at 4-under 212, followed by two South Africans — Christiaan Bezuidenhout and Branden Grace each with a 72.

Baylor School graduate Harris English (75) was tied for 72nd at 8 over.

Jordan Spieth matched the low round of the day with a 68, still seven shots behind and most likely too far back to contend with a dozen players ahead of him and complete the career Grand Slam.

Spieth was headed back to his rental home to flip on the TV, a rarity for him.

But it's Lefty. It's theater.

"I don't watch golf, but I promise you I'm going to turn it on to watch him today," Spieth said. "It's pretty incredible. I have no way to relate to it, right? But I also don't think it's necessarily that special, because didn't he win a World Golf Championships in the last couple years?

"The guy's got four good rounds on any golf course in him, and no one would bet against that."

Mickelson has had three good ones at Kiawah Island. One more for history.