The 26-year-old, who won the 2009 US Women's Open, fired a three-under-par 69 at a rain-softened Locust Hill Country Club to post a four-under total 212.
"I just keep simple things, fairway and greens because this course has really long rough," Ji said after mixing five birdies with two bogeys.
"So I just keep to the fairway and then hit into the green. My iron shots helped with my play. I'm hitting really close to the hole and I make easier putts."
Australian veteran Karrie Webb was alone in second, after carding a 68, with Italy's Giulia Sergas (69) among a group of four players knotted at two under.
Level with Sergas were American Stacy Lewis (70), former champion Suzann Pettersen of Norway (71) and South Korea's Park In-Bee (72).
After the start of the third round was delayed by over two hours due to heavy rain and unplayable course conditions, Ji made the most of her pinpoint iron play to break clear of a congested leaderboard.
Asked how she felt holding the 54-hole lead in the second women's major of the year, the Korean replied: "A little bit nervous with that, but I'm trying to keep it simple and just play my game."
Webb was delighted to recover from a bogey at the par-four first with five birdies over her final 11 holes that lifted her into contention.
"I've done my work to give myself a shot," said the 37-year-old Australian who has won 38 LPGA Tour titles, including seven Majors. "Tomorrow I just got to go out and do similar things that I've done the last couple of days and see if that's good enough."
American Paula Creamer, whose only Major victory came at the 2010 US Women's Open, was among a group of seven players locked at one under after returning a 73.
Overnight leader Pak Se-Ri of South Korea, winner of five major LPGA titles, battled to a 76 to tumble five stroke off the pace at one over par.
Taiwan's world number one Yani Tseng, who won the title last year by 10 strokes with a 19-under-par total, carded a 74 to end the day a distant 13 shots off the lead at nine over.