Thai teenager Chanettee Wannasaen has played a five-hole stretch in six under and closed with a near-flawless nine-under 63 for a four-stroke victory at the Portland Classic.
The 19-year-old became the first Monday qualifier to win on the LPGA Tour since 2015.
"I'm really proud of myself because I really worked hard for the trophy," Wannasaen said.
"I don't give up. I always work hard to improve my short game, improve my mind, improve my play. I do everything to get a trophy."
Sarah Kemp was the highest-finishing Australian, hitting four birdies and a bogey in a final-round 69 to leave her tied for 21st at 13 under.
Grace Kim (70) was a shot further back after an eventful first seven holes on Sunday, in which she hit two bogeys, two birdies and an eagle before finishing two under for the day.
Wannasaen arrived in Portland ranked 367th by the LPGA and having missed nine straight cuts in her rookie season.
But she put together four stellar rounds in the 60s at Columbia Edgewater and shattered the tournament scoring record by five strokes with a 26-under 262 for her first LPGA Tour victory.
Wannasaen is just the third qualifier to win on tour, after Canada's Brooke Henderson (Portland Classic, 2015) and American Laurel Kean (State Farm Classic in 2000).
The Thai youngster was never really threatened in the final round and comfortably beat Xiyu Lin of China, who closed with a 64 for a 22-under 266.
China's Ruoning Yin (64), American Gina Kim (66) and Spain's Carlota Ciganda (67) were joint third another stroke back..
Tied for second place entering the final round, Wannasaen charged into the lead with birdies at the third and fourth holes, an eagle at the par-5 fifth, and two more birdies at the sixth and seventh - a stretch of six under that catapulted her into the lead.
She carded a 30 on the front nine and was never really pressured on the inward nine.
She increased her advantage with consecutive birdies at Nos.13 and 14, and added another at the 17th to increase her lead to five stokes with four to play.
One of few hiccups in her final round came at the 72nd hole when Wannasaen, who hit 13 of 14 fairways, powered her approach long and over the green - one of three missed greens in regulation in the final round.
She chipped short but punctuated her bogey-free round and victory with a 10-foot par putt.