Ji, who won the 2009 U.S. Women's Open, opened her second round with back-to-back birdies only to give both shots back with a double bogey seven at the sixth after sending her tee shot into the woods.
She recovered, though, and reeled off three straight birdie threes from the ninth to sit one clear of Ko, the youngest woman to win a golf major.
Ko had begun the day three back of Ji but had picked up five shots in her first 12 holes on Friday to grab a share of the lead before mixing two bogeys with two more birdies to allow the South Korean to edge ahead.
England's Charley Hull carded a 69 to sit in tied third on seven-under with China's Lin Xi Yu (70).
Norway's Suzann Pettersen, who was accused of showing a lack of sportsmanship at the Solheim Cup last month, shot a best of the day 66 to sit tied seventh on four-under.
(Writing by Patrick Johnston in Singapore; editing by Sudipto Ganguly)
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