Japan's Ai Miyazato collected nine birdies in an eight-under par 64 on Friday to take a one-shot lead midway through the LPGA tour's season-ending Titleholders tournament.
Miyazato, seeking a third victory this season, had just one blemish on her card, a bogey at the par-five second hole.
She more than atoned, with five birdies on the front nine and four more on the back at TwinEagles for a 10-under total of 134 after two rounds.
"I was just really calm out there all day long and I did really good on the greens," Miyazato said. "My game is really in good shape but I still have a couple more days so hopefully I can keep it going."
She was one shot in front of South Korean Na Yeon Choi, the US Women's Open champion who carded a 68 for 135.
South Korea's Sun Young Yoo, part of a trio sharing the first-round lead, thought whe was two shots adrift after a 70 but was assessed a one-shot penalty for an improper drop.
That gave Yoo a one-under 71 and put her in a tie for third on 137 alongside France's Karine Icher (70), American Brittany Lincicome (69) and overnight co-leader Suzann Pettersen of Norway (71).
Yoo was penalized for failing to hold her arm at shoulder height when she took her drop, and ended up with a triple-bogey eight at the hole.
"This is one of those rules you don't like to apply a penalty to," rules official Janet Lindsay told the Golf Channel. "You don't see the player gaining an advantage, but it definitely was a breach of the rule and that is a one-stroke penalty."
South Korea's So Yeon Ryu, who also shared the first-round lead, carded a 72 to fall into a tie for seventh alongside Germany's Sandra Gal and Australian Karrie Webb on 138.
Stacy Lewis, the world number two who has locked up the Player of the Year award, was two-under through 36 holes.
Lewis, the only player to win four titles this season, needs to win the $500 000 first prize to have a chance of passing money list leader Inbee Park of South Korea.