The round was Bunch's lowest as a professional. Her previous best was a 66 at this event in 1999, a score she matched at the 2000 Subaru Memorial of Naples.
'I feel very comfortable here,' said Bunch. 'I always seem to putt really well here, and that's kind of the name of the game at this tournament. You've got to make a lot of putts; the scores are always low here. I've always putted well here and I like courses I can play aggressive on, and this is one of those courses.'
Two-time defending champion Annika Sorenstam carded three bogeys but rebounded with a birdie on the 16th and an eagle at the last to post an even-par 72.
Pat Hurst shared the lead at 8-under but bogeyed her 14th, the fifth at the Randolph North Golf Course, to fall into a tie for second. Nancy Scranton, Mi Hyun Kim and Dorothy Delasin joined Hurst at minus-seven.
Bunch, starting on No. 10 on Thursday, did most of her damage on her front side. She collected three birdies in her first four holes and added a 15-foot birdie at No. 15. Bunch made it two in a row at the par-5 16th after she chipped her third to an inch.
She reached the green in two with a 5-wood at the par-5 18th, then rolled a 20-foot eagle putt into the center of the cup for a front-nine, 7-under 30.
Bunch carded two more birdies on her back nine but dropped her only shot of the round at the sixth, where she lipped out a six-foot par save.
'I'd have to say I probably had a fairly hot putter,' said Bunch. 'To be quite honest, I didn't hit ball the ball extremely well today, but I was able to take advantage of the par-5s.'
Hurst had five birdies on her front side and ran off three in a row, starting at the first. She dropped her only shot of the round at the fifth when she missed the green short then two-putted from six feet.
Hurst had a chance to match Bunch at 64 but missed a four-footer for birdie at 18.
'I missed a couple short ones, like on the last hole, but overall, I made the putts I needed to make,' said Hurst, who hit 15 greens in regulation on Thursday. 'I hit the ball pretty consistently. I hit it close.'
Juli Inkster, the 1999 Welch's/Circle K champ, shot 66 to tie Karen Weiss and Mhairi McKay at 6-under par.
Laura Diaz, Janice Moodie, Sherri Turner and Suzanne Strudwick share ninth place at 5-under.
Full-field scores from the Welch's/Circle K Championship