Ko sits four shots behind early leaders Na Yeon Choi of South Korea and Norway's Suzann Pettersen.
The 15-year-old birdied her opening hole, the par fourth 10th, before dropping back to even par with a bogey at the 11th. Ko recorded two more birdies and a bogey to go out at one-under 35 through her front nine.
A bogey on the par four seventh saw her drop back to even par.
Ko's playing partner Michelle Wie also recorded an even par round.