Ko, 16, won twice on the tour as an amateur before turning pro. In the opening event on the tour this year, she shot a 5-under 68 and tied Meena Lee for the top spot.
The co-leaders were one shot in front of nine players, a group that included Stacey Lewis, Jessica Korda and Brittany Lang.
Ko won both the 2012 and 2013 Canadian Women's Open and her professional debut on the tour began with her making birdies on three of her first six holes at the Ocean Club.
"I was shocked that I wasn't more nervous," Ko said.