Lewis one in front of Ko at LPGA Swinging Skirts
Lydia Ko of New Zealand speaks with Stacey Lewis of the USA during Round Three of the Swinging Skirts LPGA Classic at the Lake Merced Golf Club on April 26, 2014 in San Francisco, California - by Jed Jacobsohn
America's Lewis, who won three titles last season but hasn't visited the winner's circle since her Women's British Open triumph last August, played without a bogey in her four-under 68 for a 54-hole total of 10-under par 206.
Ko had five birdies and one bogey in her 68 for 207.
It was three further strokes back to South Korea's Jenny Shin, who also posted a 68 for 210. Her compatriot Park Hee-Young carded a 68 for 211.
Lewis and Ko, who have been paired together for every round this week at Lake Merced Golf Club, were neck and neck as they traded birdies at 12, 15 and 16 before Lewis's birdie at 17 moved her back into sole possession of the lead at 10-under.
She said she wouldn't change a thing on Sunday.
"The game plan is the same. It's just trying to hit good shots and make as many putts as possible," world number three Lewis said.
France's Karin Icher, who entered the third round tied with Ko for second place, faded fast with a bogey at the second. She carded a one-over 73 that left her in a group on 212 that also included China's Feng Shanshan (68) and Thailand's P.K. Kongkraphan (70).
Ko had claimed the outright lead with back-to-back birdies at the fifth and sixth holes.
She and Lewis both parred the next four before Ko bogeyed the 11th to fall back into a tie for the lead.
Ko, ranked fourth in the world, rebounded with a long birdie putt at the 12th, but Lewis kept pace with her first birdie of the day.
The pair matched birdies again at the par-three 15th, where Ko chipped in after missing the green and Lewis drained a five-footer.
At the 16th Ko made a short birdie putt while Lewis rolled in a curling 20-footer.
Lewis got her nose back in front with a third straight birdie at the 17th, and said she was enjoying her close-up view of the maturing Ko's game.
"It was fun to watch," Lewis said. "She got going there early making some birdies and I just felt like I needed to answer."
Ko is seeking her first LPGA win as a professional, although she won her first title as a pro in Taiwan in December. She won four professional events as an amateur, including triumphs in the LPGA'S Canadian Open in 2012 and 2013.