The South Korean shot a four-under 67 to take a one-shot lead over a three-strong chasing pack consisting of Inbee Park, Mika Miyazato and Hee Kyung Seo.
"I played really well today; same as yesterday," Choi said. "I hit 16 greens and 12 fairways, so my shots were really good."
Choi is seeking her first win in four years on the LPGA tour.
"I have two more days," she said. "I don't care about the other players. I'll just try to keep my score low."
Overnight leader Pernilla Lindberg (71), Hee-Won Han (67), Karine Icher (69) and So Yeon Ryu (68) were 7 under.