Mickelson was tied for the 36-hole lead and made a brilliant start to round three, with four birdies in his first seven holes seeing the American seize a five-stroke advantage at one stage.

The back nine proved more challenging - the 12th and 13th holes especially, where the left-hander followed a bogey with a double - but Mickelson was still able to card a two-under-par round of 70.

Mickelson is seven under for the tournament in South Carolina, the second men's major of the year.

Fellow American Koepka also made 70, with a bogey on the 18th preventing him from sharing the 54-hole lead.

South Africa's Louis Oosthuizen is a further shot back in third, after an even-par round.

All of the top three are major champions - Mickelson has five to his name, including the Professional Golfers' Association of America's (PGA) flagship event in 2005, Koepka four and Oosthuizen one.

Koepka, who is still not fully recovered from knee surgery, won the PGA Championship in 2018 and 2019.

Just 12 players are under par at Kiawah Island, but among them are a pair of Americans who have won a major before, Bryson DeChambeau and Gary Woodland.

Both are five shots behind Mickelson.

The PGA Championship concludes tomorrow.