Americans Jame Park (after 13 holes) and Marissa Steen (15) were co-leaders on six under par
Ko, the world number one for 69 straight weeks, has gone into the 2017 season with recent coaching, caddie and equipment changes, and with Jutanugarn threatening her top ranking.
Adding to the intrigue, the top two golfers now have the same coach - South African Gary Gilchrist.