The South Korean shot a 5-under 67 to finish at 16-under 272 at the tricky Locust Hill course. The 22-year-old, in her second year on the tour, had a previous best finish of second to Pettersen at South Koreas Kolon Championship in October.
Pettersen shot a 72 and surrendered a three-stroke lead in the final round. Ji got in front for the first time with a 7-foot birdie putt on No. 15, where Pettersen missed a 5-footer for par.
The 28-year-old Norwegian is a five-time tour winner in 2007.