Phil Mickelson takes a one-shot lead into the final round of the US PGA Championship as he bids to become the oldest major winner following a remarkable Saturday at Kiawah Island.

But that advantage was wiped out as Brooks Koepka and Louis Oosthuizen drew level before dropping shots late on.

Koepka sits one back at six under, with Oosthuizen at five under par.

Five-time major winner Mickelson eventually signed for a two-under-par 70 to keep himself in pole position on seven under par.

He won the last of his majors at the 2013 Open at Muirfield and victory this week would see him take the record of oldest winner from Julius Boros, who won this title in 1968 at the age of 48.

“I’m enjoying it and playing well, I made some good swings, played some good shots and I’m really having a lot of fun,” said Mickelson.

“I let my mind lapse on a couple of swings and this course will penalise you for that.”

Fellow American Kevin Streelman is at four under, with South Africans Christiaan Bezuidenhout and Branden Grace at three under par.

US Open champion Bryson DeChambeau double-bogeyed the 17th but finished with a birdie to leave himself five adrift at two under par, alongside fellow American Gary Woodland.

England’s Paul Casey made three birdies but they were cancelled out by four bogeys in a one-over 73 that leaves him at one under par heading into the final round.

Countryman Matt Fitzpatrick sits one further back as part of a large group gathered at level par that includes Americans Jordan Spieth and Rickie Fowler.

Northern Ireland’s Rory McIlroy, who won the event the last time it was staged at this venue in 2012, posted a two-over 74 to slip to five over par.

Lefty provides the drama

Mickelson turns 51 next month but looked in the form of his life as he took just 32 shots for the first nine holes.

The 2005 winner teed off with a share of the lead on Saturday as he became the only player to hold top spot after a major championship round in the 1990s, 2000s, 2010s and 2020s.

He was soon alone at the summit after Oosthuizen bogeyed the opener, before superb approach shots at the second and third holes set the platform for successive birdies as Mickelson extended his lead to three.

That became four shots thanks to a birdie at six and, with Koepka, Grace and Masters champion Hideki Matsuyama all making their move in the chasing pack, Mickelson restored his advantage with another birdie at seven.

The left-hander, who dropped out of the top 100 in the world earlier this year for the first time since 1993, has not posted a top 10 in a major since the 2016 Open Championship.

But he continued to defy his ranking by making a tremendous par after finding a waste bunker beside the ninth green as he reached the turn at four under par and extended his advantage to five shots with a birdie at the 10th.

But then Kiawah Island began to show its teeth.

The Ocean Course was built to host the Ryder Cup in 1991, the same year Mickelson won his first PGA Tour title as an amateur at the Northern Telecom Open.

The American’s winning total now stands at 44 and should he make it 45 this weekend, Mickelson would become the first man to win a PGA Tour event three decades after his first.

That task got harder after he made his first bogey in 21 holes at the 12th at the same time as Oosthuizen birdied for a two-shot swing.

Further drama unfolded on the 13th when both players leaked their tee shots into the water. Mickelson dropped two shots after having to re-play his third shot from the tee, while Oosthuizen escaped with a bogey to cut the lead to one stroke.

Koepka brings the competition

Four-time major champion Koepka, playing in the group in front, started the day one back and found himself five off the pace after Mickelson’s early charge.

But the American had wiped out that deficit by the time he headed to the 17th tee.

However, he said his putting “was the worst I’ve ever had in my career”.

He added: “Anything inside 10 feet is usually running three or four feet by and then my putt on the last barely got to the hole, so I have to work on that.”

The former world number one suggested before the tournament he was six months away from full fitness after having knee surgery in March, but despite his struggles on the green, he once again showed his immense presence in major championships.

Birdies either side of an uncharacteristic par miss from close range at six and another dropped shot at nine saw the 2018 and 2019 US PGA champion hit the turn in level par and at four under overall.

Koepka picked up shots at 10 and 12 and then a birdie at the par-five 16th drew him level after the leaders stumbled in the pairing behind.

The usually unflappable Oosthuizen showed signs of frustration as he failed to convert a birdie putt on the 16th that would have taken him into a three-way share of the lead.

A bogey followed at the par-three 17th for the South African as Mickelson held his nerve to come through unscathed.

With Koepka bogeying the 18th to sign for a two-under-par 70, Mickelson was back on top and he made a smart up-and-down at the last to maintain that lead heading into Sunday’s final round.

Spieth eyes unpredictable Sunday

Spieth needs to win this event to become just the sixth man to complete the career Grand Slam, but his attempts to join Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus, and Tiger Woods in that elite club faltered with opening rounds of 73 and 75.

That left the American at four over par heading into the weekend but the world number 28 gave himself an outside chance of chasing down the leaders with a superb 68 on Saturday.

Spieth made the most of the calm early conditions to post five birdies through 16, before his lone bogey of the day followed on the par-three 17th as he finished at level par.

“I made a couple of putts but, man, I struck the ball really well for the first 16 holes,” he said. “I was proud of the way I hung in there and got myself back to level par to give myself a shot of getting into the top 10.”

The 27-year-old also predicted the South Carolina venue, which is not only the longest course in major championship history but also one that is exposed to unpredictable winds coming off the Atlantic with 10 oceanside holes, will pose a different challenge on Sunday.

“I think it will feel like a different course,” he said. “Every hole will play very different. I think that’s really cool.”