After struggling at last week's Scottish Open, Ko again had issues with her putting, having taken 65 putts in total through the first two days, negating a decent day with the driver and some strong iron play, hitting 16 of 18 greens in regulation.
It was a consistent day, with Ko carding a bogey-free round, but several of her contemporaries went lower to set a tough pace to follow at the top of the leaderboard.
Ko is 10 shots behind Korean leader In-Kyung Kim, who shot a 68 to create a two shots lead over England's Georgia Hall and American Lexi Thompson.
Kim is a two-time winner on the LPGA Tour this year, and is one of the most in-form golfers on the tour, but will be challenged by Thompson, who has risen to a career high second in the world rankings.
Some of women's golf other leading lights are struggling, with world number one So Yeon Ryu alongside Ko at one-under par, while world number three Ariya Jutanugarn missed the cut.