Fighting her way into a tie at the end of 72 holes and then shutting the door on her rival, Ryu won the US Women’s Open on Monday, defeating Hee Kyung Seo by three shots in a three-hole playoff.

Ryu became the fifth South Korean to win the Open and the fourth in the last seven years. Yes, they take their golf seriously there and Se Ri Pak — the queen of the sport in that country — was out on the course watching the 21-year-old Ryu make history.

“I’m proud of it,” Ryu said. “To see my hero, Se Ri Pak, out there fighting for me. It’s pretty powerful, yeah.”

Ryu played the three-hole playoff in 2-under par, all but sealing it when she hit three perfect shots to the green on the par-5 17th for a birdie while Seo drove into a bunker and had to scramble for bogey.

For good measure, Ryu hit her approach on 18 to four feet for another birdie.

But really, it was the birdie she made on that hole about an hour earlier that set her up for the win.

Trailing by one to an opponent who had closed out her round before darkness stopped play the previous night, Ryu hit a 6-iron uphill, over the lake on No. 18 to six feet. She slammed the putt home to pull into a tie and ended up with two birdies in an hour on a hole that yielded only 28 over five days.

Nobody will ever say Ryu backed into this victory, won on a 7,000-yard Broadmoor course that got hit by storms every day, turning it into a marathon for some players and a sporadic series of starts and stops for others.

Seo was the best on Sunday, when she played 36 holes over 14 hours and finished both rounds at 3-under 68. But there was one hiccup: A short putt that rimmed out on No. 17 when she was rushing to finish — a ball hit while the wind was whipping, leaving her uneasy as she stood over it. It left her at 3 under instead of 4 under and gave Ryu a glimmer of hope.

“I think one mistake yesterday on the 17th green, that’s the one,” Seo said.

She scrambled through the back nine, saving par with a tricky 5-foot putt on 11, again from an awkward stance above a greenside bunker on No. 13, then again after a drive into the deep rough on 15.

“I just trusted myself and just let it go, and I made lots of birdies,” Seo said.

Angela Stanford briefly pulled into a tie with Seo, but missed a three-foot putt for bogey on No. 11 to start a free-fall — 4-over par on holes 11 through 14.

By the time Seo reached No. 17, she was ahead by two, pointing and staring at a rainbow overhead.

Most of the players arrived at the course around 5 a.m., and when they got there, they were looking at two of Japan’s best players — Mika and Ai Miyazato — in the lead.

Some 15 hours later, and thanks to a pair of third-round 76s from the Miyazatos, Japan’s flag had been replaced by South Korea’s at the top of the leaderboard and the Miyazatos — unrelated but both from Okinawa — were 3 over with five holes left.

Seo and Ryu are part of a half dozen or so South Koreans fondly known as the Seoul Sisters, who are trying to take over where five-time major winner Se Ri Pak left off, adding their names to the long list of successful players from a country that produces plenty.

With an eye trained for fashion and a swing built for winning, Seo has also earned another nickname — “Supermodel of the Fairways.”