Chun shot a three-under-par 69 in round two at Maryland's Congressional Country Club, a more sedate 18 holes than the opening round but still enough to grow a record-equalling five-shot lead to six.

The two-time major winner had her first minor wobble of the event, bogeying holes seven and eight, but responded with a blemish-free back nine that also featured two birdies.

Chun, from South Korea and twice a major champion, is now 11 shots under par for the tournament.

Sixty-nine was only two shots off the best score of the day, with Ko and Thompson among four players to shoot 67.

New Zealand's Ko, the Tokyo 2020 bronze medallist chasing a first major victory since 2016, produced five birdies on the front nine and is now five under for the tournament, tied for second place.

Kupcho, the American who won the season's first major, the Chevron Championship, and picked up a second career Ladies Professional Golf Association Tour victory last weekend, is tied for second place with Ko after a 68.

Past Women's PGA Championship winners Brooke Henderson from Canada, Kim Sei-young of South Korea and Australian Hannah Green are in the group an additional shot back and tied for fourth place.

Americans Jennifer Chang and Caroline Inglis join them.

Thompson, who holed out from more than 100 yards for an eagle two on the 17th hole en route to a round of 67, is at three under, eight shots off the lead, and joined there by Australian Minjee Lee.

Lee won the US Women's Open earlier this year, while Thompson has 15 top-10 finishes at majors since her lone victory in 2014.