'I'll buy every single putter I ever play with if I can putt like that,' Kerr, a nine-time tour winner, said of her 26-putt round at the tree-lined Locust Hill course. 'I feel like my game is coming around.'
Canada's Alena Sharp, showing steady progress in her third year on tour, ran off seven birdies and two bogeys to shoot 67.
She was one better than Norway's Suzann Pettersen, who is still glowing after capturing her first major title at the LPGA Championship two weeks ago.
'I don't think I'm going to realize it (happened) until the end of the year when it's time to relax and sit back,' Pettersen said. 'Just want to build on the momentum, try to get the game better and better each day. That'll keep you busy.'
Top-ranked Lorena Ochoa, who won here in 2005, was tied for fourth at 3-under with Brittany Lincicome and South Korea's Mi Hyun Kim.
Defending champion Jeong Jang carded a 71, while 50-year-old Hall of Famer Nancy Lopez, who is playing a limited schedule this year, slumped to an 82.
A mid-afternoon thunderstorm stopped play for a little more than two hours. Sandwiched between two majors -- the U.S. Women's Open is next week at Pine Needles in Southern Pines, N.C. -- the $1.8 million tournament drew 46 of the top 50 money winners.
Absent was Annika Sorenstam, whose best finish in four tries was second in 1996, and Stacy Prammanasudh, who pulled out Thursday with a thumb injury.
Starting on the back nine, Kerr opened with three birdies, sinking a 25-foot putt on No. 11. She slotted in another 25-footer on No. 17, bogeyed the next hole after driving into trees, then rolled in three more birdies after the turn.
The 29-year-old Miami native has earned $7 million since joining the tour in 1997. After playing poorly on the greens all season, she said a 'really bad putting day' at a tournament in Korea in May convinced her to try a Ping Craz-E.
'It's the putter that Karrie Webb used to revive her career last year,' Kerr said, although she couldn't explain why it's made such a difference. 'It's just sometimes you need a new look and a new feel.'
She followed up early this month with a third place at the Ginn Tribute, her best finish this year.
Kerr said she began feeling sick Sunday night, took antibiotics but still 'felt really crummy' Wednesday. She teed up after getting 9 hours of sleep.
'I really didn't feel that poorly today,' she said. 'I'm just carrying a lot of tissues in my bag.'
Sharp has had her best season since turning pro in 2005, earning $68,151 in 11 starts. She missed the cut in five tournaments but notched a career-best tie for 11th at the SBS Open in February.
'I know I'm a good enough player to be in the top 30 out here,' she said. 'It's just a matter of believing in it.'