Irish rookie Leona Maguire sank nine birdies on her way to a seven-under 65 to take the lead after the opening round of the LPGA Tour's Mediheal Championship.
Jasmine Suwannapura of Thailand birdied Nos. 15 and 18 to make a late run up the leaderboard, finishing alone in second at six under at Lake Merced Golf Club.
Albane Valenzuela of Switzerland and Alison Lee are tied for third at four under.
Maguire felt the Northern California course was reminiscent of those she is used to in Ireland. Maguire's first professional win in the US came in 2019 at a Symetra Tour event in the same part of the state.
"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."
Maguire, 26, came close to winning her first LPGA Tour title in April when she tied for second at the Lotte Championship.
She started her first round Thursday on No.10 and produced six birdies on her opening nine holes. She could have turned in a 64 had she not bogied her final hole of her day, the par-5 9th.
"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." – Leona Maguire
"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," Maguire said.
Lee wound up carding a double-bogey on No.14, only to regain both those strokes back the very next hole with an eagle on the par-5.
Valenzuela had former Cardinal teammate Emily Wang on her bag.
"She just graduated yesterday from her master's degree in international policy and she's a Rhodes scholar," Valenzuela said. "So, she is probably, have to say, the smartest caddie of the week because she outsmarts everyone."
World No.2 Inbee Park, a seven-time major champ from South Korea, is part of a six-way tie for fifth at three-under 69.
Lexi Thompson opened with a one-under 71 four days after her back-nine collapse at the US Women's Open.
Joining her at one under is Su Oh, who are both six behind the lead in a tie for 14th.
Oh is the best of the Australian contingent, one ahead of 2019 Women's PGA Championship winner Hannah Green.