Ko became the youngest winner in LPGA Tour history last year when she won this tournament at age 16. She also won the New South Wales Open in Australia when she was 14.
She shot a 5-under 65 Thursday and was tied after one round at the Royal Mayfair Golf Club with Christel Boeljon and Angela Stanford.
Cristie Kerr and Paula Creamer shared fourth place at 66 while world No. 1 Inbee Park was at 67 along with Na Yeon Choi, Karine Icher and Jennifer Kirby. Park has six victories this year, three of them at major championships.
Ko birdied four of the first seven holes and suffered a lone bogey at the par-4 15th.
She shot a 13-under 275 last year to win the tournament when it was played at the Vancouver Golf Club. This tournament was last played at Royal Mayfair in 2007, when it was won by the now-retired Lorena Ochoa.