Killeen, 42, of Marietta, Ga., opened with six consecutive pars before posting an eagle-3 on the 481-yard 16th hole and adding four birdies.
'That just kind of jump-started everything,' she said of the hole.
After that, she used precision iron play to set up short putts for birdies on Nos. 3, 5, 6 and 9, getting a good start on a self-imposed goal of 18 birdies for the tournament.
She did so while fighting humid, 87-degree heat and the biting greenhead flies that sometimes make life miserable on the Seaview Marriott Resort course, which sits next to a saltmarsh.
'If I could get through 18 holes without getting bitten by those bugs, I'd be happy,' Killeen said.
'They're just little devils with wings,' said Cristie Kerr, who was one shot back at 5 under.
Sergas, 24, of Trieste, Italy, started well and stayed that way, drilling a 20-foot putt for birdie on her first hole and adding three consecutive birdies to overtake Kerr, already in the clubhouse.
But she knows it will take more to win.
'Of course, I'm really excited about it. But I know this game. I didn't do anything yet. This is just the start,' Sergas said.
Sergas, who posted three consecutive birdies on her back nine, has never won an LPGA event. Her best finish was 22nd place.
Five players - Brandie Burton, Dawn Coe-Jones, Johanna Head, Kristal Parker-Manzo and Gloria Park - were all at 4 under after the opening round of the 54-hole event, which runs through Sunday.
Kerr, the LPGA tour's fourth leading money-earner, is the highest-ranked player in this year's Classic field. Annika Sorenstam opted not to play the event.
Kerr was happy to be in contention but was dissatisfied with her tee shots, so she returned to the driving range after her round to practice.
'To not play your best golf and shoot 5 or 6 under is pretty encouraging. Some days it's like that,' Kerr said.
Coe-Jones and Burton, who played in the same threesome, used solid iron play and consistent putting to keep their scores down.
'A lot of our putts were within 6 feet, 7 feet today,' Coe-Jones said. 'And sometimes you can feed off the other player in your group, if they are playing well. And Brandie and I, I think we both did that.'
Head, who has never won an LPGA title, also eagled the 16th and had two birdies after that.
Defending champion Angela Stanford struggled, missing numerous short would-be birdie putts en route to a 5-over par 76.
The shot of the day came from Tracy Hanson, who carded a hole-in-one on the 113-yard, par-3 13th hole using a pitching wedge.
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