Ireland's Leona Maguire posted an outstanding opening seven-under-par 65 to take a one-shot lead at the Mediheal Championship LPGA event in California.
The 26-year-old from Cavan, who started at the 10th, carded nine birdies and two bogeys for a 65.
Maguire leads Thailand's Jasmine Suwannapura by one with Switzerland's Albane Valenzuela and US player Alison Lee sharing third three off the pace.
"I suppose first arriving it felt like home," Maguire said.
"It's a lot of the same style of golf course, same weather as home. Got a nice feeling straight away when we arrived on Monday."
But while Maguire prospered at Lake Merced Golf Club, Northern Ireland's Stephanie Meadow only had three players behind her in the 144-strong field after her opening 80.
Maguire came close to winning her first LPGA Tour title in April when she tied for second at the Lotte Championship and is being touted as a possible member of this year's European Solheim Cup team.
After starting at the 10th, she produced six birdies on her opening nine holes and she could have signed for a 64 had she not bogeyed the final hole of her day.
"I birdied my opening two holes, which kind of settled me in quite nicely, holed some really good putts and in general it was a pretty stress-free day," added the Irishwoman.
Maguire's good form this season has seen her move up from 177th to 93rd in the world rankings.
World number two Inbee Park is part of a six-way tie for fifth after a 69 with Thailand's Patty Tavatanakit, winner of this year's ANA Inspiration major, carded a 2-under 70.
Lexi Thompson opened with a one-under 71 four days after her back-nine collapse at the US Women's Open.