Teenager Yuka Saso modelled her golf game after Rory McIlroy, spending hours watching videos of his swing before going to bed each night in order to perfect her own.

All that work paid off and now Saso is a US Open champion just like her idol thanks to a clutch playoff putt after a back-nine collapse by Lexi Thompson.

Saso's 10-foot putt for birdie on the third playoff hole on Sunday helped her edge out Nasa Hataoka to become the second teenager to win the US Women's Open.

Saso overcame back-to-back double bogeys early in the round to make the playoff and then became the first player from the Philippines to win a golf major.

"I was actually a little upset," Saso said about her poor start. "But my caddie talked to me and said, 'Just keep on going; there's many more holes to go.' That's what I did."

Both players made pars at Nos. 9 and 18 in the two-hole aggregate playoff, sending the tournament to sudden death back at the ninth hole.

"I don't know what's happening in the Philippines right now, but I'm just thankful that there's so many people in the Philippines cheering for me," she said.

"I don't know how to thank them. They gave me so much energy. I want to say thank you to everyone. I'd like to thank my family, I wouldn't be here without them."

Saso matched 2008 winner Inbee Park as the youngest US Women's Open champion at 19 years, 11 months, 17 days.

Thompson, who had a five-stroke lead after the eighth hole, played the final seven holes in five over to finish a stroke back at three under.

The only other players to finish under par on the Lake Course at Olympic Club were Megan Khang and Shanshan Feng, who both were at two under.

High school junior Megha Ganne played in the final group but shot 77 and finished three over as the low amateur for the tournament.

"I'm going to remember this for the rest of my life," Ganne said. "It's everything I've wanted since I was little, so it's just the best feeling."

Saso has talked frequently about her time studying McIlroy's swing and the four-time major winner sent Saso an Instagram message before the final round imploring her to get the trophy.

"I saw it this morning, and I was like, 'Ohhh!'" she said. "I should have reposted it but I was so busy this morning, so I'll do it later. I felt really happy."

Saso got the trophy after a rough start to the final round with double bogeys on the second and third holes that seemed to knock her out of contention. She managed to steady herself with a birdie at No. 7.

Saso then made back-to-back birdies on the par-5 16th and 17th holes to get to four under and join Hataoka in the playoff.

Hataoka used a run of three birdies in a four-hole span on the back nine that put pressure on Thompson, who wilted down the stretch.

"It's hard to smile, but it was an amazing week," said Thompson.

Australians Minjee Lee (14 over) and Hannah Green (17 over) both shot four-over 75s and finished tied 54th and 62nd respectively.