The players were forced off due to lightning in the late afternoon but after returning to the course, Choi and Webb overhauled the clubhouse lead jointly held by Mika Miyazato and 17-year-old amateur Min Lee.
Choi and Webb each had six birdies in blemish-free rounds, with Choi starting strong before Webb came back at her, tying it up with a birdie on the 17th as they each carded a 65.
Miyazato recovered from two bogeys on the opening nine with five birdies on the way back, including three in a row on the 15th, 16th and 17th.
Lee likewise had two early bogeys but also had seven birdies, including the 16th and 17th, to match Miyazato.
Sun Young Yoo also came through to match their 66s, recording four consecutive birdies between the 13th and 16th holes.
American Cristie Kerr was part of a group tied for eighth place, three shots off the lead.