Pornanong Phatlum fired an 8-under 64 on Thursday to grab a 1-stroke lead after the opening round of the Safeway Classic.
Phatlum's lone LPGA Tour title was an unofficial event in Brazil last year.
Lexi Thompson is one stroke off the lead at minus-7 at Columbia Edgewater Country Club.
Thompson's Solheim Cup teammates, Cristie Kerr and Lizette Salas, share third place at 6-under 66. They were joined there by Sandra Gal.
World No. 2 Stacy Lewis carded a 5-under 67 and was one 14 players tied for sixth place. Among those she was tied with were Juli Inkster, Michelle Wie, Yani Tseng and Ai Miyazato.
Defending champion Mike Miyazato, no relation to Ai, opened with a 1-under 71 on Thursday and was tied for 76th.
Phatlum started on the back nine in the afternoon wave and was chasing Thompson, who started early on the back nine. Phatlum flew up the leaderboard with four birdies in her first five holes.
The 23-year-old stumbled to a bogey at the par-3 16th, but atoned for that mistake with a birdie at the 18th.
After birdies at one and three, Phatlum faltered to another bogey at the fourth. She erased that error with a birdie on the fifth. Phatlum followed with birdies at six and seven to move one clear of Thompson.
Phatlum closed with pars on eight and nine to lead by one.
"I changed my putter this week and it is feeling very good. I made many birdies today," said Phatlum, who had just 22 putts in her round. "I was putting not good last week and I felt not very good with the old putter, so I just changed."
Thompson got her round going with a birdie on the 10th, her first. She converted back-to-back birdie chances at the 13th and 14th.
After moving to 4-under with a birdie on No. 16, Thompson ran off six straight pars from the 17th.
Thompson, who shot 81 in the second round here last year and missed the cut, birdied the par-5 fifth to move to minus-5. She then eagled the par-5 seventh to grab the early lead.
"I hit a lot of good iron shots and I played the par-5s in 4-under, so I took advantage of those. Overall, I've just been working on the consistency of my game and working on tightening my shots up," Thompson admitted. "Obviously, being close to the lead after the first round, you try to take that momentum into every other round after that. But it's golf so you have to take one shot at a time and not get ahead of yourself."
NOTES: The first round was completed despite a one hour weather delay at the start of the day ... Natalie Gulbis had a hole-in-one on the par-3 second, for her seventh career ace in competition ... Paula Creamer withdrew before the round due to personal issues, while Jin Young Park withdrew during her first round.