Mickelson, a five-time major winner who last won the PGA Championship in 2005, surged home with five birdies in his last eight holes for a three-under 69.

It put the 50-year-old five-under for the championship, good enough to share the lead with 2010 British Open champion Louis Oosthuizen.

Mickelson is looking to become the oldest player to win a major, besting Julius Boros who won the 1968 PGA Championship at 48.

"Right now there's a lot of work to do ... But 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 said.

South African Oosthuizen was one clear of his American rival for much of the late afternoon until a final-hole bogey left him on 68 and share of top spot.

Four-time major winner Brooks Koepka also found his way to six-under during the afternoon thanks mainly to two stunning eagles, but bogeys in two of his last four holes saw him card a 71 and sit third at four-under, just one back.

Masters champion Hideki Matsuyama is chasing the rare feat of winning back-to-back majors to start the year, not done since Jordan Spieth won the Masters and US Open in 2015.

After sharing the lead at the start of his round, Aussie Cameron Davis struggled on Friday. PHOTO: Stacy Revere/Getty Images.

The Japanese star shot 68 to get to three-under where he was joined by South African pair Branden Grace (71) and Christiaan Bezuidenhout (70).

The Australian tilt took a huge body blow after round one pacesetter Cameron Davis imploded.

Davis showed poise to rebound from a triple bogey in round one but was unable to do the same after a first-up double bogey in round two.

The 2017 Australian Open champion started on the 10th hole of Kiawah Island's Ocean Course in a share of first after overnight leader Corey Conners produced five bogeys in his opening six holes.

But a loose drive followed by a layup into thick grass conspired against the Sydney native and he was quickly demoted.

Worse was to come as bogeys around the turn preceded a second triple of the tournament, making an eight on the par-five second hole.

His 78 left him three-over.

Cameron Smith shot 73 to sit at one-over for the tournament, six adrift of the lead as the best Australian.

He remained confident of a weekend charge.

"A score is still out there, which I love," Smith said.

"The course has been set up really nice. It's just you've got to kind of get lucky and get some of those longer putts to go in."

Former World No.1 Jason Day looked almost certain to miss the cut, and the chance to earn a US Open berth, after a 75 left him five-over.

But high winds destroyed the afternoon wave and the cut line moved three-shots from when he signed his card to earn him a reprieve.

Lucas Herbert (72) Jason Scrivener (75) and Matt Jones (75) are four-over.

Two of Australia's biggest names – Adam Scott (72) and Marc Leishman (76) – missed the cut by a shot with Leishman lamenting a double bogey, bogey finish.

- Ben Everill