The 22-year-old from Perth, who won the women's PGA Championship in June, is joined at the top of the leaderboard by South Korea's Mi Jung Hur who also fired a 64.

Americans Jane Park and 18-year-old qualifier Yealimi Noh were one shot back (65) while defending champion Marina Alex was at two under (70).

Starting at the 10th, World No.26 Green birdied both back-nine par-5s and turned at four-under before storming home with birdies on her final three holes.

Fellow Aussies Katherine Kirk, Sarah Kemp and Su Oh are tied for 52nd after shooting two under (70), while Robyn Choi is one stroke further back (71).

Top-ranked Jin Young Ko saw her record run of 114 consecutive holes without dropping a shot come to an end in Oregon, finishing at four under (68) and currently four shots off the lead.

RIGHT: Ko played 114 consecutive holes before dropping a shot. PHOTO: Jonathan Ferrey/Getty Images.

The South Korean, a four-time winner this year including two major championships, had not made a bogey since early in the third round of the Women's British Open, where she finished third.

Her streak eclipsed Tiger Woods' PGA Tour record of 110 bogey-free holes, set in 2000. It's also the longest known bogey-free streak in LPGA Tour history.

Ko was proud of her achievement, but the run was wearing on her.

"I did 114 holes and then done. It's cool," Ko said.

"So I want to do again, 115 holes bogey-free round hopefully. But yeah, I'm free."

Joanna Klatten of France enjoyed the shot of the day making a hole-in-one on the 167-yard 13th hole.