KIAWAH ISLAND, S.C. — Phil Mickelson was cruising.

And then he wasn’t.

Mickelson, at age 50 trying to become the oldest major championship winner in the history of the sport, looked like he was en route to duplicating what Rory McIlroy did in 2012 in the PGA Championship on Kiawah Island’s Ocean Course when McIlroy ran way and hid from the rest of the field in a record eight-shot victory.

Then Mickelson did a couple of Mickelson things on the back nine, throwing away three shots in two holes as a result of two poor tee shots and watching what was a five-shot lead fizzle and dissolve faster than an antacid pill in a glass of water.

Mickelson, who shot a 2-under-par 70 in Saturday’s third round and is at 7-under for the tournament, recovered from his bogey on No. 12 and double on No. 13 in time to maintain a one-shot lead entering Sunday’s final round.

The problem, however, may be the person on whom he holds the one-shot lead: Brooks Koepka.

Koepka, the 2018 and 2019 PGA Championship winner who looks like he’s very well recovering from the knee surgery he underwent in March, is a natural-born killer. He also shot 70 on Saturday and is 6-under.

Louis Oosthuizen, who played with Mickelson on Saturday and shot 72, is two shots back at 5-under. Kevin Streelman is 4-under, Christiaan Bezuidenhout and Branden Grace are both 3-under. And Bryson DeChambeau, Gary Woodland and Joaquin Niemann are 2-under.

While Mickelson had been getting all attention and adulation this week, Koepka, like one of the many alligators in the weeds on this golf course, has been lurking on the leaderboard all week, never making any dramatic moves, but staying within striking distance.

Here’s the scary thing about Koepka: He hasn’t once sounded like he’s played very well all week.

“I left a lot out there,’’ Koepka said after his round Saturday. “I’ve got a chance to win, so that’s all I wanted to do today, not give back any shots and be there [Sunday] with a chance, and I’ve got that.’’

Then Koepka called his third round “the worst putting performance I think I’ve ever had in my career. Can’t get much worse.’’

He worked on his putting on the practice green until dark after the round. So, too, did Mickelson, who cut his post-round interview short so he could go out and hit balls and putt.

Mickelson had this tournament by the throat after he birdied No. 10 and got to 10-under with a five-shot lead. Then, that straying focus that he’s spoken about of late bit him on 12 and 13.

The tee shot snap-hooked into a lagoon on 13 came right after Louis Oosthuizen, his playing partner, dunked his tee shot into the lagoon.

“Phil hit two bad tee shots and cost him three shots,’’ Oosthuizen said. “Other than that, he played beautifully. He putted well. He drove it unbelievably long and straight. I think we all got lucky that he came backwards into the field.’’

Mickelson said, “Those two swings were more a product of not staying or keeping the feel and the focus of the shot. And so that’s just a small little thing that I need to iron out.’’

Before that, he was in a zone.

“I felt I had a very clear picture on every shot, and I’ve been swinging the club well, and so I was executing,’’ he said. “I just need to keep that picture a few more times. Even though it slipped a little bit, and I didn’t stay as focused and as sharp on a few swings, it’s significantly better than it’s been for a long time.

“I’m making a lot of progress, and I’ll continue to work on that and hopefully I’ll be able to eliminate a couple of those loose swings [Sunday], because I’m playing a lot better than the score is showing and I think if I can just stay sharp, I’ll post a score that is … that better reflects how I’m actually playing.’’

A score that results in his sixth major championship.