Morgan Pressel fired record-equalling eight-under par 64 on Friday to join Lee Mi-Rim atop the leaderboard after two rounds of the LPGA Kia Classic.
America`s Pressel had eight birdies with no bogeys in a round that matched the course record at Aviara Golf Club.
Her 10-under par total of 134 tied her for the halfway lead with South Korea`s overnight leader Lee, who signed for a second-round 69 that included six birdies and three bogeys.
They were one stroke in front of Americans Alison Lee, who climbed up the leaderboard with a 66, and Cristie Kerr, who carded a 68.
Spain`s Maria Hernandez also posted a six-under 66 to grab a share of fifth place on eight-under 136. She was tied with Japan`s Sakura Yokomine (67), South Korean Kim Hyo-Joo (68) and American Brittany Lang (68).
World number one Lydia Ko of New Zealand headed a group on 137, firing a two-under 70 for her 26th straight LPGA Tour round under par.
She was tied with world number three Stacy Lewis, who shot 69, and Alison Walshe, who carded a 67.
Pressel described her sparkling effort as "stress free."
"I hit almost every fairway, and I think I hit 16 or 17 greens and just had a chance for birdie on almost every hole," she said.
"I just hit it where I wanted to hit it today. It`s been a while since I`ve been confident knowing where the golf ball would go, and so it was nice to swing freely out there today and have the result that I did," added the American, who won the most recent of her two LPGA titles in 2008.
"It`s hard to win out here," Pressel said. "Sure, you would like to win more. But I`ve put a lot of time and hard work in."
Ko, the 17-year-old sensation who became number one in the world in February, has made winning look easy in her short, spectacular career.
Her 26th straight under-par round has her closing in on Swedish great Annika Sorenstam`s LPGA record of 29 set in 2014.
"I don`t know all the records and all the details on that, but like I always say, I`m going to try and have fun," said Ko, who has two wins worldwide this year. "If I play good golf and break some records, great."