Choi turned in a four-birdie, one-bogey round for a 3-under-par 68 and a 54-hole total of 13-under 200.
Inbee Park closed with six birdies in the final seven holes Saturday in moving to second place at 11-under. Karrie Webb, after matching Choi's third-round 68, is in third at 9-under while Ai Miyazato (68 Saturday) and Suzann Pettersen (70) are tied for fourth at 8-under 205.
Park charged into contention after a round in which she started with 11 pars. She started her run with birdies at 12 and 13 and jumped up the leaderboard with four consecutive birdies to finish the round.
Choi, who said she played "safely" Saturday, had a birdie at the first hole and another at No. 5. She slipped with a bogey at 10 but birdied the par-5 12th hole for the third consecutive round and stretched the lead with a birdie at 16.
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