The 25-year-old South Korean's five under par round contained six birdies and one bogie, moving her level with Lewis at the head of the field on a rainy day in the island republic.
"I started very well and I felt great," Choi said.
"The weather wasn't good until we finished, but we can't control that. I was sitting down inside and talking about non-golfing things with my caddie, just trying to think positively.
"I feel very excited about Sunday and I feel comfortable with the situation and this position, so it should be really exciting."
Lewis, who had led by two shots at the tournament's midway stage, started her third round with two bogies in the first three holes before rallying with five birdies to maintain a share of the lead on 14 under par.
Lewis' compatriot Paula Creamer was on her own in third place, two shots off the lead, with four players a further three shots behind.
Defending champion Angela Stanford improved slightly, carding a 73 in her third round. However, after 76s in her opening two rounds she was out of contention, 23 shots off the pace ahead of the final day's action.