Lee had four birdies in a span of five holes late in her round and leads Brittany Lang by one stroke. Lang's 6-under 65 included a bogey on the last hole after she was 7-under in the first 10.
Paula Creamer, Jodi Ewart Shadoff, Ji Eun-hee ad Beatriz Recari are tied for third at 5-under and eight other players are at 4-under.
Defending champion and world No. 1 Inbee Park had two bogeys and three birdies Thursday and is tied for 28th at 1-under 70.
Lee, with one of the earlier tee times, opened with a birdie and made the turn at 3-under after birdies at Nos. 7 and 9. She ran off three consecutive birdies over 13-15 and a birdie at the par-3 17th left her at 7-under.
Lang started on the 10th hole Thursday and opened with four consecutive birdies and birdies at 15 and 16 had her at 6-under after seven holes. A birdie at the first hole pulled her into a tie with Lee. But she had seven consecutive pars before the final-hole bogey 5 at the ninth and had to settle for second.