China's Lin Xi Yu (67) was one shot back in second, with American Morgan Presell (68) in a four-way tie for third alongside Germany's Sandra Gal, Charley Hull of England and Yoo Sun-young of South Korea.
"I was just patient with the winds and I was calm with my putting. I just looked at the line and hit it,” Ji told reporters, crediting some short game work for her good round.
"I just working on my short game last week and the week before, because my chipping wasn't feeling good.
"So I was working a lot this week and I just chipped in twice today and I think I worked on a lot of right things."
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Ji, who won the 2009 U.S. Women's Open, has struggled this season, with her best finish tied for eighth at the Evian Championships last month. The 66 was a joint best round of the year.
World number two Lydia Ko also had few problems in the tricky conditions as she fired four birdies in a round of 69, which was good enough for a share of seventh.
Home favourite and former world number one Yani Tseng could only manage a level par 72 after encountering trouble on the front nine, but that was five better than American world number three Stacey Lewis, who endured a miserable birdie-free 77 -- her worst of the year.
(Writing by Patrick Johnston in Singapore; Editing by Peter Rutherford)
