Phil Mickelson of the United States walks on the 18th green during the third round of the 2021 PGA Championship at Kiawah Island Resort's Ocean Course on Saturday in Kiawah Island, South Carolina. Photo: VCG

Phil Mickelson had an iron grip on the PGA Championship until two bad swings changed the entire complexion of the tournament on Saturday and left "Lefty" facing a more daunting challenge in his quest to become golf's oldest major winner.

Mickelson had been almost flawless in the third round until he pulled his drive into a bunker at the par-four 12th and made his first bogey in 21 holes.

That, coupled with a Louis Oosthuizen birdie, cut the lead from five to three strokes.

There is potential disaster on every hole on Kiawah Island's back nine, and another poor swing with a two-wood by Mickelson at the par-four 13th was never headed for anything other than the watery grave right of the fairway.

Shockingly, the error came right on the heels of Oosthuizen finding the same hazard moments earlier.

"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," said the 50-year-old.

"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 [of the backswing] ... and I end up flipping it.

"I felt I had a very clear picture on every shot and I've been swinging the club well, and so I was executing. I just need to keep that picture a few more times."

Mickelson's ball appeared never to have crossed the hazard line, which meant he had to re-tee and hit his third from the same spot.

Oosthuizen, however, took a penalty drop up on the fairway after both players agreed that the South African's ball had been in play before flying across the hazard line into the water.

Overhead TV video was not definitive, however, and Mickelson said Oosthuizen had acted properly.

"His ball clearly crossed over the hazard. I didn't think that was an issue," Mickelson said.

"Mine was on the edge, and I just didn't feel good about it. As much as it hooked, I just didn't think it crossed."

Mickelson double-bogeyed the hole, while Oosthuizen made bogey.

"Phil hit two bad tee shots and cost him three shots. Other than that he played beautifully," said Oosthuizen, who shot even-par 72 to end the round in third place, two shots behind Mickelson and one back of Brooks Koepka.

"He putted well," Oosthuizen continued of Mickelson.

"He drove it unbelievably long and straight. I think we all got lucky that he came backwards into the field."

Their media obligations complete, Mickelson and Oosthuizen both went to the driving range to work on their swings in the fading light.

Bryson DeChambeau was the only other player there.