Pettersen birdied four of her last seven holes to post a 13-under total of 131, ending the round level with Colombian Mariajo Uribe (66), who has yet to win on the LPGA Tour.
Cristie Kerr, joint leader overnight with fellow American Cheyenne Woods and Thailand's PK Kongkraphan, was a further shot back after carding a 69 while Taiwan's former world number one Yani Tseng (65) was among a group of seven players at 11 under.
Cheyenne, the 24-year-old niece of 14-times major winner Tiger followed her opening 63 with a 73 to head into the weekend at Whistle Bear five strokes off the pace.
Pettersen, who has recorded three top-10s on the LPGA Tour this season without adding to her career tally of 14 wins, was delighted to maintain good form with next week's KPMG Women's PGA Championship in Westchester, New York approaching.
"Why I am playing well is hard work over time," the Norwegian told Golf Channel after hitting all 18 greens in regulation during the second round.
"I am starting to kind of put the puzzles together.
"I'm really looking forward to next week so this is a good test. I am hitting the drives really good and long which is nice and that puts me in a good position for the par-fives. If you play this course well, I think par is five or six under.
"I decided to go out, be patient and hit a lot of good shots," said Pettersen, who has claimed two women's majors -- the 2007 Women's PGA Championship and the 2013 Evian Championship.
South Korea's world number two Park In-bee, the defending champion, was at seven under after carding a 68 while top-ranked Lydia Ko of New Zealand was a further two strokes back, also after a 68.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue/Steve Keating)
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