Ko ended 2015 with her debut major title in the Evian Championship and the ANA was the first of five majors the players would challenge this season.
"I've just got to have fun and that is the big key for me in any event, just enjoy to it.
"Sometimes I am going to have average days and those days I just laugh it off and go hey there's another round it's not the end of the world.
Ko was one under after nine and repeated that on her second nine after a mixed collection of three birdies and two bogeys.
Other players nibbling at the lead are Catriona Matthew, Lee-Anne Pace and Shiho Oyama who all shot four under par rounds of 68.
Defending champion Brittany Lincicome and the 2014 champion Levi Thompson have yet to tee off in their afternoon pairing.