The Norwegian picked up five birdies in the opening six holes and finished one shot ahead of Sweden's Karin Sjodin.
A further two strokes back are Ai Miyamoto, Azahara Munoz, Ryu So-yeon, Moon Hyun-hee and Kim Ha-neul, who all carded rounds of 66.
''Being three rounds, you want to get off to a hot start and I can't complain with today's round,'' said Pettersen.
''I got off to a good start, found the feel of the greens this morning and worked hard to feel the speed yesterday and made some great putts and great shots.
"I'm really trying to read what's out there; some pins are par pins and some pins you attack.''
World number one Yani Tseng was five strokes back after carding a 68.