Inbee Park shot a six-under 66 to take the first-round lead in the LPGA's Kia Classic at Aviara, just one week out from the ANA Inspiration - the season's first major.
Park had a solid, bogey-free round in rainy, breezy conditions in the afternoon.
Fellow South Korean Hyo Joo Kim was a stroke behind, with Sophia Popov, In Gee Chun, Stephanie Meadow and Mel Reid shooting 68.
Park made four birdies on the back nine to complete a flawless opening round.
Her late birdie combined with Kim's misstep on the 18th paved the order after 18 holes.
Kim had an erratic day, carding eight birdies but bogeying two of her last four holes.
"Today I also made a lot of mistakes that I shouldn't have made, and (Friday) I'm looking forward to not making as many mistakes and having a great round," Kim said.
The players are tuning up for the ANA Inspiration, the season's first major, which will be played next week at Mission Hills in Rancho Mirage.
Australia's Minjee Lee and Sarah Kemp both opened with even-par 72s to be in a tie for 29th, six behind Park.
Both had almost identical rounds with three birdies and three bogeys each, but on different holes.
Some of the focus was also on 31-year-old Michelle Wie West, who was playing in her first event since giving birth to daughter Makenna nine months ago.
This is her first tournament since the Women's PGA Championship at Hazeltine in June 2019, when she shot 84-82.
She had hoped for a smoother return to the tour but finished the round tied with Yani Tseng for 142nd in the 144-player field.
Wie West opened with a nine-over 81. She birdied the first and then carded seven bogeys in her next 13 holes. She made a double bogey-6 on No. 15 and closed with a bogey on 18.