The Norwegian made four birdies in the last six holes to press home her advantage after a slow start.
Pettersen did not get her first birdie until the par five seventh and followed it up with a bogey on the par three eighth.
Last year's US Women's Open champion So Yeon Ryu is currently second after a solid if unspectacular round where she birdied the third, fifth and seventh before dropping a shot on the eighth.
Another birdie on 12 was followed up with four consecutive holes before another she made another bogey on the penultimate hole to leave Pettersen out in front.
"It just felt like I got out there this morning and I didn't really expect it to be as breezy as it was," Pettersen told the LPGA website.
"It kind of took me a few holes to kind of adjust, and it almost took me nine holes to kind of adjust to the conditions.
"I made a good birdie on seven, missed a very short putt on eight, a bit of a silly one, and I kind of lost my concentration, and I said to myself, you can't let your focus slip. As much as I wanted to make it, it was almost a break point where I got my focus back, it got my attention."