Paired together in Saturday's final grouping, Yang answered every time Lewis tried to trim her three-shot deficit.
Yang, who shared the lead after 54 holes last year, opened a four-stroke lead at the 13th before Lewis capitalized on a two-stroke swing at the 14th, closing within two. But Lewis had a three-putt bogey at the 17th en route to a 69, giving Yang a three-stroke advantage heading into the final round. Yang, at 8-under 202, and Lewis, at 5-under 205, will be paired again Sunday.
In Gee Chun, playing in her first U.S. Open, shot a 2-under 68. The 20-year-old South Korean is alone in third place at 4-under 206. Japan's Shiho Oyama is fourth at 3 under after a 71.
Defending champion Michelle Wie played through hip and ankle pain, firing a 68. She is in a four-way tie for fifth at 2-under 208, along with two-time winner Inbee Park (70).
South Korea's Chella Choi shot the first nine-hole score of 29 for the championship. But the 24-year-old missed a three-foot putt on her final hole that would have tied the championship's single-round scoring record. She settled for a 6-under 64, shooting up the leaderboard into the pack at 2 under.
Brooke Henderson of Smiths Falls, Ont., moved into a tie for 18th after shooting 68 on Saturday. She entered the final round at 1-over 211.