Choi had six birdies and a bogey Friday and moved to 10-under 134 after 36 holes.
Miki Saiki, whose round Friday included eagles on two par 4s, had a second-round 66 and is alone in second at 9-under. First-round co-leader Morgan Pressel was 2-under on the day and is two strokes behind Choi.
World No. 1 Inbee Park, seeking to be the first player to win four major tournaments in a year, was 1-over in the second round and is 2-under for the tournament, eight shots behind Choi.
Camilla Lennarth, who shared the lead with Pressel after Thursday's play, suffered through a 10-over 82 Friday. Her round included one birdie, seven bogeys and a quadruple-bogey 8 at the 17th hole.
That pushed her to 4-over for the tournament and she missed the cut by two strokes.
Saiki had two bogeys and four birdies Friday to go with her pair of eagles. She holed out an 8 iron from 127 yards at the fourth hole and dropped in a wedge shot from 108 yards at the seventh.
Choi had birdies on three of her first six holes but bogeyed No. 7. She answered with a birdie at the eighth and added others at 12 and 15 for her 67.