Weather conditions were poor throughout the day, with fog and rain early and stiff winds later. Only five players were able to beat par Thursday.
Pettersen's scorecard included bogeys at Nos. 2 and 16 with six birdies in between. Irene Cho, Caroline Hedwall, Karine Icher and Alison Walshe are tied for second at 1-under.
Cho, Icher and Walshe each had three birdies and two bogeys Thursday. Cho's birdies came in a three-hole stretch. Hedwall battled through a round in which she had five birdies and four bogeys.
Pettersen was 1-over after four holes but moved under par with birdies at Nos. 5 and 6. She pulled clear of the field with three straight birdies to open the second nine and added a birdie at the par-4 15th before the bogey at 16.