She fired a season-low and tournament record 9-under-par 63 in Saturday's third round to move to 20-under 196.
"I'm just trying to play with my feel and first instinct," Pettersen said. "I have to go out and do my job tomorrow. It was nice I could actually extend my lead with people pushing me, shooting 66. Tomorrow's not going to be that easy."
Laura Davies (66) is second at 13-under 203.
Paula Creamer (66) is third at 12-under 204, and three players are tied for fourth at 9-under 207.
"Suzann's playing really well," Davies said. "She’s gone mad. A 6-under is a good round, just not good enough this week."
Defending champion Hee-Won Han (72), who is playing for the first time since the birth of her first child this summer, shares 16th place with Maria Hjorth (68) at 5-under 211.
Members of Congress to keep receiving porn magazine
Rosie O'Donnell unveils nearly 50-pound weight loss