The South Korean at one point made seven birdies in eight holes to race into a lead which ties the biggest ever seen at a women's major after the opening round.

Chun carded an eight-under 64 to leave herself in a great position from which to attack a third career major victory.

Chun, who started on hole number 10, went birdie-birdie-birdie-birdie to close the back nine.

The 27-year-old's lone blemish came on the par-four first, which she bogeyed, but three consecutive birdies followed.

By contrast, the two players tied for second place made only four birdies combined.

Chun's compatriot Choi Hye-jin, playing in her first Women's PGA Championship, made three in a bogey-free round of 69.

Pornanong Phatlum of Thailand also went bogey-free, with one birdie and a hole-in-one eagle on the second hole.

American Jennifer Chang and Paula Reto from South Africa are six shots off the lead in a tie for fourth place.

Just 14 players finished the first round under par - accentuating the brilliance of Chun's golf - but the last three winners of this event were in that group, all shooting 71.

Nelly Korda of the United States, the defending champion and also the reigning Olympic champion, said "I don't know what golf course In-gee is playing" after insisting she was happy with a 71 on a long course.

A double bogey on the par-four 12th hampered 2020 champion Kim Sei-young, especially as it was followed by two bogeys on the next three holes, but the South Korean fought back admirably.

Australian Hannah Green, winner in 2019, shot four birdies in a up-and-down round.

American Jennifer Kupcho, the Chevron Championship winner earlier this year, is also at one under, while New Zealand's Olympic bronze medallist and two-time major winner Lydia Ko is among those on even par.

The four-round tournament is due to reach its climax on Sunday (June 26).