Emerging from a midseason slump, Tseng had eight birdies and a double bogey at The Vancouver Golf Club. The Taiwanese star opened the season with victories in three of her first five events, then had a series of missed cuts and poor finishes before tying for 11th last week in Oregon in the Safeway Classic.
U.S. Women's Open champion Na Yeon Choi was second.
Lydia Ko, the 15-year-old South Korean-born New Zealander who won the U.S. Women's Amateur two weeks ago, was two strokes back at 68 along with South Korea's Inbee Park. In January, Ko won the New South Wales Open in Australia at 14 to become the youngest player to win a professional tour event.
Brittany Lincicome, the winner last year at Hillsdale in Mirabel, Quebec, opened with a 72. Michelle Wie, the 2010 champion at St. Charles in Winnipeg, Manitoba, had a 74.