The Korean, who had gone into the day level with Karrie Webb on six under, carded 67 to move clear at the summit.
Choi twice recorded three birdies in the space of four holes - on the third, fifth and sixth and on the back nine at 10, 12 and 13.
Dropped shots at the 11th and 17th opened the door slightly for the trailing pack but nobody could get closer than American Sydnee Michaels, who shot 65 to move to eight under.
Suzann Pettersen burst into contention with a 64, with five of her seven birdies coming in an outstanding back nine.
After a level-par first round, that left the Norwegian sharing third place with Lizette Salas of the United States and Japanese pair Momoko Ueda and Mika Miyazato.
Webb remained at six under alongside fellow Australian Lindsey Wright, Scotland's Catriona Matthew, American Paula Creamer and Sun Young Yoo of South Korea.