Lewis had five birdies and two bogeys for a 12-under 132 total at the Siam Country Club Pattaya Old Course, replicating her form from 2012 that made her the first American to win the LPGA Player of the Year award since Beth Daniel in 1994.
Thai teenager Ariya Jutanugarn shot a bogey-free 66 in windy and humid conditions to move into second place. Beatriz Recari (68), So Yeon Ryu (68) and Catriona Matthew (69) were tied for third at 8 under.
Two-time defending champion Yani Tseng (68) recovered from a 75 on Thursday for a share of 25th place.
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