After failing to fire at the Canadian Open, the Kiwi 20-year-old has slipped two places down the world rankings to seventh.
Ko failed to make the weekend at the Canadian Women's Open - a tournament she won three times, including twice as an amateur.
It extended her drought to seven straight tournaments without a top-10 finish. It comes after Ko had accrued seven top-10 finishes in her previous 10 tournaments.
Ko began the year at number one and held the position until June, ending 84 weeks at the top spot. She hasn't won a tournament since the Marathon Classic in July last year.
So Yeon Ryu remains the world number while Sung Hyun Park has moved to third behind American Lexi Thompson.