First-round leader Sophie Gustafson posted a 2-under 70 and stands one stroke behind Pak at 8-under-par 136. Michelle Ellis, Jamie Hullett, Suzann Pettersen and Wendy Ward share third place at 5-under-par 139.
Pak bogeyed the first and 18th at Lincoln Hills Club during an opening-round 71. She began her second round on the back nine and drained back-to-back birdies from the 11th. She later hit a sand-wedge to 15 feet for birdie at the par-5 15th to help her make the turn at minus-4.
On the front side, she converted a four-foot birdie at the first. She followed with a long two-putt birdie at the next and made it three straight when she sank a two-footer for birdie at the third.
Her hot streak was halted with a bogey at the par-3 fourth. She responded from that mistake by draining a 50-foot birdie putt at the seventh that got her back to 7-under. She followed by knocking a fairway-wood within 18 feet of the cup to set up eagle on the par-5 eighth to grab first place.
'My swing feels great and I am hitting my driver really well,' said Pak, who's 64 matched the 18-hole tournament record set by Kelly Robbins and Amy Fruhwirth in 1996 at the Twelve Bridges Golf Club. 'Thursday was kind of a struggle off the tee. So I practiced in the afternoon after my first round. I feel much better now.'
Pak, who has won three times this season and 21 times overall on the LPGA Tour, is still chasing her goal of finishing number one on the money list, but knows Annika Sorenstam will be hard to catch.
'One of my goals every year is to try to be number one,' said Pak, who trails Sorenstam by $404,878 on the money list. 'My offseason goals are still there, but last week Annika made it harder (with her victory). I still have six more tournaments left, so there's still a chance.'
Gustafson got on the board when she rolled home a 25-foot birdie putt at the third. She missed the green at the par-4 fifth and that led to her lone bogey of the day.
She came right back with a birdie on the sixth and closed out her scoring when she drained a five-foot birdie putt at the 16th.
'I didn't play as half as good as I did on Thursday,' said Gustafson. 'I didn't hit many fairways, which meant that I couldn't reach any of the par- fives. That also made it much harder to get it close.'
Karen Stupples fired a 5-under 67 to climb into a share of seventh place. She is joined at 4-under-par 140 by Patricia Baxter-Johnson, Beth Daniel, Tracy Hanson, Pat Hurst, Gloria Park, Grace Park, Michele Redman, Rachel Teske, Karrie Webb and Jung Yeon Lee.
Defending champion Cristie Kerr heads a group of nine players at 3-under-par 141 after a round of 3-under 69.
The cut line fell at 3-over-par 147 with 80 players advancing to the final two rounds.