Ji will seek her second career title after her finishing 54 holes at 4-under par 212, good for a one-shot advantage over Karrie Webb at Locust Hill Country Club in Pittsford, N.Y.
The 26-year-old's only previous title was at the 2009 U.S. Women's Open.
Seven-time major champion Webb fired a 4-under 68 to move into second place at minus-3.
Third-ranked Stacy Lewis mounted a challenge with a round of 2-under 70, forging a third-place tie with world No. 5 Suzann Pettersen (71), Inbee Park (72) and first-round co-leader Giulia Sergas (69) at minus-2.
Tumbling from first to 15th place was Se Ri Pak, who carded a 4-over 76, ending at plus-1.
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