Lewis had three birdies on each side, moving to 11-under 133 after 36 holes.
Six players are tied for second at 9-under. Choi Na-yeon and Ariya Jutanugarn each matched Lewis' score of 66. Choi had a bogey and seven birdies and Jutanugarn posted six birdies.
First-round leader Azahara Munoz was 2-under 70 and among those tied for second. Also at 9-under for the tournament are Chella Choi and Paula Creamer, who both shot 67 in the second round, and Yoo Sun-young, who had a 68.
Lewis birdied three par 4s in the first six holes, including back-to-back birdies at Nos. 5 and 6 that moved her into the lead. She added birdies at the par-5 12th and the par-3 14th and 17th holes.