The South Korean birdied three of the last four holes to reach 15 under at the Atlantis Resort's Ocean Club course and she leads American Lizette Salas by a single stroke.
Salas, who is still winless on the LPGA Tour, enjoyed a bogey-free round of 66 while Paula Creamer (71) and Jessica Korda (72) both reached 12 under by eagling the 18th hole.
Creamer may well have been out in front come Saturday night had she not suffered a triple bogey on the 15th hole.
Stacy Lewis, Amelia Lewis and Jenny Suh are not far behind with Michelle Wie and teenager Lydia Ko part of a large group on 10 under.
Sixteen-year-old New Zealander Ko is making her first start as an LPGA Tour member and found three birdies and a bogey in her round of 71.