AP photo by Chris Carlson / Phil Mickelson tees off on the seventh hole of the Ocean Course during the second round of the PGA Championship on Friday in Kiawah Island, S.C.

KIAWAH ISLAND, S.C. — The short jab with his left fist to celebrate birdies and even a few pars. A thumbs-up to the gallery. Phil Mickelson at times looked to be about the only one having fun Friday in a PGA Championship that has become the ultimate test without being extreme.

But then, what's not to enjoy?

The 50-year-old Mickelson looked like the Mickelson of old on another windswept grind around the Ocean Course at Kiawah Island, running off five birdies over his last eight holes, the last one giving him a 3-under-par 69 that held up for a share of the 36-hole lead.

"To know I'm playing well heading into the weekend, to be in contention, to have a good opportunity, I'm having a blast," said Mickelson, who has the 2005 PGA Championship among his five major championships and lacks only a U.S. Open victory for the career Grand Slam.

Mickelson shared the lead with Louis Oosthuizen, the 38-year-old South African with the sweet swing and hard luck in getting that second major title. The 2010 British Open champion didn't make a bogey Friday until his final hole, and his 68 allowed him to join Mickelson at 5-under 139.

It was the highest 36-hole score to lead a PGA Championship since the last time at Kiawah Island in 2012.

The opportunity for Mickelson includes his bid to become golf's oldest major champion — the record was set by 48-year-old Julius Boros in the 1968 PGA Championship — and to show he can still beat the best in the world.

Mickelson has not won on the PGA Tour in two years, though last year he did win his first two starts as a golfing senior on the PGA Tour Champions. His most recent major championship was the 2013 British Open. He no longer is among the top 100 in the Official World Golf Ranking.

But he's Phil Mickelson, and Lefty has spent a career leaving fans wondering what he'll do next.

"I think he has the bit between his teeth," said three-time major champion Padraig Harrington of Ireland, who played alongside Mickelson for two days. "I think he believes he can do it in these conditions. He's not here (just) to make the cut."

Mickelson is the oldest player to have a share of the 36-hole lead in a major since Fred Couples, who was 52 at the 2012 Masters and wound up tying for 12th.

Brooks Koepka had a pair of eagles offset by four bogeys and scrambled for par on the 18th hole for a 1-under 71 that left him one shot behind the co-leaders — and in conditions he loves.

"It's a major, man. It's going to be tough, especially with the wind blowing," said the 31-year-old American with two PGA Championship victories (2018-19) and a pair of U.S. Open titles (2017-18). "It doesn't matter, just go out and go play."

South Africa did its part to stock the upper portion of the leaderboard, with Oosthuizen joined in contention by countrymen Christian Bezuidenhout (70) and Branden Grace (71), who were tied for fourth at 3 under with Masters champion Hideki Matsuyama of Japan, who dropped a shot on the 18th hole but still shot a 68.

The cut was at 5 over, and Baylor School graduate Harris English made it on the number despite a bogey on his final hole of the round, the par-4 ninth. English offset six bogeys with four birdies for a 74 that had him tied for 63rd.

Only 18 players remained under par, which included 2020 U.S. Open champion Bryson DeChambeau, who looked exhausted walking off the course after a 71 that featured no birdies on the back nine. Still, he was tied for 12th at 1 under.

The casualties included the top two players in the world — Dustin Johnson and Justin Thomas missed the cut — and a tee marker on the 17th that Erik van Rooyen smashed when his shot went into the water. The head also came off the South African's club.

There were so many more examples of players having reasons to lose their mind.

Cameron Tringale was two shots off the lead going to the 14th hole and 15 shots behind when he walked off the 18th green. That included a bogey, double bogey, triple bogey, quadruple bogey and quintuple bogey, not in that order. He rallied for an 82.

Shane Lowry hit one so far to the right on the par-5 16th that he was on the beach. A picket fence in his way, the Irishman was able to get back on grass and saved par on his way to a 71.

"It's not very enjoyable out there because it's so hard, and every hole is a disaster waiting to happen," said Lowry, the reigning British Open champion. "So it's very stressful and there's a lot of anxiety and a lot of nerves and a lot of tension out there, but you just have to get on with it and try and hit the best shots you can, and that's all I've been doing."

Ian Poulter was 6 under for his round through 12 holes when he noticed a video board behind the green that suggested he had a shot at the course record. It's a wonder Poulter's eyes didn't pop out of his head.

"I just started laughing to myself like, 'Who in the world would write that and put that on a board with that last five holes to play?'" said the 45-year-old Englishman whose best major finish was a tie for second at the 2008 British Open.

He bogeyed four of his last six, which feature the four hardest holes on the course, for a 70.

"Every single shot you hit, you have to be focused and diligent and not take anything for granted. It's a piece of work," England's Paul Casey said after a 71 that left him part of the five-way tie for seventh and three shots out of the lead. "But I quite enjoy it in a sick and twisted kind of way."

Grace had a bogey-free round and was in the lead at 6 under until he hit his tee shot into the water on the par-3 17th and made a double bogey, and then he made a bogey on the closing hole. Mickelson was being interviewed on TV when Grace fell back with his double, and this development immediately was conveyed to Mickelson with dramatic effect.

Lefty was not overly excited.

"If you were to tell me that Sunday night, I'd really enjoy that," he said. "But right now there's a lot of work to do. ... The fact is I'm heading into the weekend with an opportunity, and I'm playing really well and I'm having a lot of fun doing it."

Mickelson has shown glimpses of improvement in recent weeks, but he is concerned about losing focus. This had his attention. He also has a 2-wood in the bag that helps him control his accuracy, at least with the wind at his back. He missed only three fairways.

"If he can keep it straight and hit it the way that he's been hitting, he's going to be around on Sunday for sure," said Australia's Jason Day, the 2015 PGA Championship winner who made the cut on the number this year. "With Phil, you kind of get some off-the-map drives that make it very interesting, and he's kept it very, very straight over the last two days."