Ko and Shin each turned 3-under-par 70s in the third round and are at 17-under 202 with one round to play. Beatriz Recari shot 2-under 71 Saturday and is in third at 11-under.
Five players, including world No. 1 Yani Tseng, are tied for fourth at 9-under. Also in that group is second-round leader Mariajo Uribe, who had seven bogeys in a 6-over 79 Saturday.
Ko, still an amateur, is looking for her second title among the professionals in as many weeks. She won her home country's New Zealand Women's Open last week.
She had birdies Saturday on No. 2, 4 and 6 and got to 4-under a birdie at the 10th hole. She dropped strokes at 12 and 14 but got even with Shin with an 18th-hole birdie.
Shin, 24 and in her eighth professional season, was also 3-under after six holes, posting a birdie at the fourth hole and holed a shot from the bunker for an eagle at the par-5-sixth. She had a birdie at No. 15 but a bogey at 17 gave Ko a chance, with the birdie at 18, to tie for first.