Choi finished 36 holes at 9-under-par 133, one shot better than a trio of competitors. She seized the advantage with back-to-back birdies from the 15th, then parred the last two holes to close the day atop the leaderboard.
Inbee Park, meanwhile, fired 6-udner 65 to gain a share of second place at minus-8 with Hee Kyung Seo (66) and Mika Miyazato (68).
Miyazato carded a bogey on the 18th, costing her a share of the second-round lead.
Tied for fifth at minus-7 were first-round leader Pernilla Lindberg (even-par 71), 2011 U.S. Women's Open champ So Yeon Ryu (68), Hee-Won Han (67) and Karine Icher (69).