PATTAYA, 27 Feb 2015 (AFP) - Lewis' perfect round, marked by eight birdies, puts the American 14 under par for the tournament.
She is followed by the consistent Yang who struck a round two 66 to return to the clubhouse at 11 under, with Thai home favourite Ariya Jutanugarn leading a clutch of players on eight under.
Ariya, who experienced heartbreak in 2013 when losing her nerve on the final hole to be edged into second place, shot a solid 69 to stay within striking distance of the overnight leader.
It was however a disappointing day for Taiwan's Tseng Yani who shared the lead at the start of the day but dropped five shots to go round in 72 (six under) -- despite an eagle on seven.
Fellow day one leader Brittany Lang eased off the pace with an error-strewn round, which came apart with a double bogey on 11 that left her on five under.
She is level with compatriot Michelle Wie who struck a superb 66 to edge back into tournament after going round in 73 on the first.
Defending champion Anna Nordqvist heads into the weekend at two under after a poor back nine undid a morning of good which saw her on course to go round in under 70.
Meanwhile, world ranked number two Inbee Park is well out off contention after making four bogeys in another disappointing day for the Korean star.
Leading second-round scores from the $1.5 million Honda LPGA Thailand at the Siam Country Club on Friday:
130 - Stacy Lewis (USA) 66-64
133 - Amy Yang (KOR) 67-66
136 - Ariya Jutanugarn (THA) 67-69, Lee Mi-Rim (KOR) 67-69, Caroline Masson (GER) 70- 66 , Jenny Shin (KOR) 70-66
137 - Cristie Kerr (USA) 68-69, Sandra Gal (GER) 71-66, Catriona Matthew (SCO) 71-66
138 - Tseng Yani (TPE) 66-72, Lee-Ann Pace (RSA) 71-67
139 - Michelle Wie (USA) 73-66, Brittany Lang (USA) 66-73, Budsabakorn Sukapan (THA), 67-72, Lexi Thompson (USA) 68-71, Feng Shanshan (CHN)70-69, Mo Martin (USA)68-71, Azahara Munoz (ESP) 69-70
Selected:
144 - Inbee Park (KOR) 70-74