World number 14 Atthaya, an LPGA rookie 19-year-old making only her fifth tour start, fired the low round of the week at eight-under-par 64 at Aviara Golf Club in Carlsbad, California.

That left her deadlocked with Madsen after 72 holes on 16-under 272 and, after missing a six-foot birdie putt for the win on the first playoff hole, Atthaya took advantage of Madsen going into the water and claimed the crown with a bogey on the second.

"I just focus on my game and do the best I can and it has turned out to be (enough)," Atthaya said.

"It means a lot. It means the world to me. I know my parents and coaches back home are watching me now. It's meaningful to all of us, not just me."

South Korean An Na-rin was third on 273 with Canada's Maude-Aimee Leblanc, Thailand's Pajaree Anannarukarn, and top-ranked Ko Jin-young of South Korea on 274. England's Charley Hull was seventh on 276.

Ko fired a 68 for her LPGA-record 34th consecutive under-par round.

The event was the final tuneup for next week's first major women's golf tournament of the year, the Chevron Championship at Rancho Mirage.

The playoff began at the par-4 18th hole. Madsen sent her approach over the back of the green while Atthaya bounced her second shot six feet beyond the hole.

But Madsen chipped the ball two feet from the cup and Atthaya pushed her birdie putt wide left to set up another playing of 18.

After Madsen chipped her fourth onto the green, Atthaya putted to eight feet. Madsen missed her bogey putt and the teen two-putted for the win.

In the final round, Atthaya made nine birdies, matching the lead with birdies at the par-4 16th and par-5 17th and sinking a clutch eight-foot par putt at 18 to stay level with Madsen, who had three holes to play.

The Dane drove over the 16th green, chipped to three feet but then missed her birdie putt as An tapped in for birdie to join the leaders.

Madsen found dirt left of a cart path at 17 but blasted her approach to eight feet and sank the tense birdie putt to reclaim the lead alone as An lipped out on a three-foot par putt to fall back.

At 18, Madsen suffered her first three-putt bogey of the week, missing a six-foot par putt and making a closing 70 that forced the playoff.

Madsen became the first Danish player to win an LPGA event by winning in Thailand two weeks ago in her 93rd career tour start.

Madsen began with a three-stroke lead and birdied the first and par-5 fifth, then made 11 straight pars before her birdie-bogey finish.

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© Agence France-Presse