The South Korean bogeyed the 15th hole and then had an even worse time on 16 at Maryland's Congressional Country Club, taking an unplayable lie on her way to a double-bogey seven.

Those holes undermined what had until then been an even-par round, with Chun carding a three-over 75 to drop to eight strokes under par for the tournament.

Compatriot Choi Hye-jin - making her Women's PGA Championship debut - and American Lexi Thompson both shot two-under rounds of 70 and are three shots back, tied for second.

Kim Sei-young, who won this event in 2020, is also tied for second.

Kim made just one bogey en route to a third-round 71, Thompson shot up the leaderboard with three birdies in four holes, beginning on 12 and all coming on par fours, and Thompson's playing partner Choi also finished strongly with a bogey-free, two-under back nine.

Thompson has 15 top-10 finishes at majors since her lone victory in 2014, Chun is a two-time major winner, Kim's 2020 Women's PGA Championship victory is her sole major win and 22-year-old Choi's best finish at a major is third place at this year's US Women's Open.

Australia's Hannah Green, winner of the Women's PGA Championship in 2019, is four under in fifth place.

Canadian Brooke Henderson, the 2016 champion, and Jennifer Kupcho, the American who won this season's Chevron Championship, are in a group five shots off the lead.

Reigning US Women's Open champion Minjee Lee from Australia is a further shot behind in a tie for 10th place.

The final round is scheduled for tomorrow.