The world No. 3 enjoyed four straight birdies from on the front nine holes 4-7, plus four more on the back nine, to produce a two-day total of 14-under 130.
"It really was a pretty easy day," Lewis said. "I didn't cause myself too much stress."
Yang shot five birdies and eagled the par-5 10th in a 66 to lie at 11-under overall.
Four players were another three shots back at 8-under, Germany's Caroline Masson, South Koreans Jenny Shin and Mirim Lee, and Thailand's Ariya Jutanugarn.
Former two-time champion Yani Tseng of Taiwan, an overnight leader with Lewis and Brittany Lang, carded 72 to drop to 6-under, while Lang shot a 73 to be at 5-under.