Yoo had nine birdies while Ryu had seven and Pettersen shot a bogey-free round in earning shares of first place.
Pettersen had the most consistent round Thursday. She was 2-under after 11 holes and then had back-to-back birdies at 12 and 13 and birdies at 16 and 18 for her 6-under round.
Ryu's lone bogey came at the par-4 10th and she had four birdies the rest of the way, including at 13 and 14 -- both par 5s -- and 18.
Yoo was 3-under after seven holes but gave back all those strokes with a double bogey 5 at the 8th hole and a bogey at No. 9. She opened her second nine with a birdie before running off three straight birdies over Nos. 12-14. Like Pettersen, she had birdies at 16 and 18.
Astronomers offer more expansive view of universe
McPhee, Cokas 'working on their marriage' after affair