Before the 70th U.S. Women’s Open, if anyone outside the Republic of Korea had been asked about In Gee Chun, there would have been a lot of blank stares. But the golfer who was voted most popular by fans of the Korean LPGA Tour and most influential by its media is no longer a mystery outside of her home country.

The 20-year-old with the effervescent smile and indomitable spirit produced one of the championship’s more improbable final-round performances on Sunday at Lancaster Country Club. Chun’s 4-under-par 66 over the 6,288-yard layout, which included three consecutive birdies from No. 15, was enough for a one-stroke victory over countrywoman and 54-hole leader Amy Yang.

For the 25-year-old Yang, it was her second runner-up finish and the fifth time since 2010 that she has placed among the top 10.

“It didn't go like what I expected,” said Yang, who missed a 10-foot par putt on the 72nd hole that would have forced a three-hole playoff with Chun.

Chun, who posted a 72-hole total of 8-under 272 to match the 72-hole scoring record by Hall of Famers Annika Sorenstam (1996) and Juli Inkster (1999), joined 2005 champion Birdie Kim, Kathy Cornelius (1956) and inaugural winner Patty Berg (1946) as players who have claimed the Harton S. Semple Trophy in their first attempt.

Not only is she the third-youngest champion behind countrywomen Inbee Park and Se Ri Pak, she also is the seventh different Korean (Park won it twice) to win the Women’s Open since Pak’s breakthrough victory in 1998.

Back home, where the sun was just starting to rise on the work week (Korea is 13 hours ahead of EDT), the victory resonated with golf fans, who adore Chun’s style and gregarious nature. Nicknamed “Dumbo,” she has a legion of fans who call themselves the “Flying Dumbos,” and a large group was expected to greet her in Seoul upon her return from the Women’s Open.

While she owns seven KLPGA Tour victories – three this season – and claimed the 2015 World Ladies Championship Solonpas Cup, a major event on the Japan LPGA Tour, winning the Women’s Open is Chun’s biggest accomplishment.

“I still don’t realize I won this championship,” Chun said through a translator. “Ever since I started golf, the fan support I've got in Korea was something that I could not have done without. So all those fans have led me up to this stage [and] I really want to thank the fans.”

With record crowds that exceeded 134,000 for the week, Chun seemed oblivious to the noise and hoopla. If she made a bogey, like she did on No. 10 on Sunday, it was on to the next shot. Even walking to the 18th tee with a one-stroke lead, she was high-fiving spectators and smiling as if she was playing a practice round.

“Her calmness is very similar to Jiyai [Shin],” said Australian-born caddie Dean Herden, who guided Shin to three major titles and was on the bag for So Yeon Ryu’s U.S. Women’s Open playoff win in 2011 at The Broadmoor. “She goes to the next tee not even thinking about what happened on the last hole. From a caddie’s point of view, it’s a luxury because there is no emotion there.”

Starting the day four strokes behind Yang, Chun was barely in the discussion. The focus was on the final pairing of Yang and world No. 3 Stacy Lewis, a two-time major champion with five top-3 finishes in 2015, including a playoff loss at the year’s first major championship, the ANA Inspiration. Lewis, who started three strokes back, birdied the first hole and had the look of someone ready to make it a match-play duel. She appeared to be the overwhelming favorite among the gallery.

But other players joined the fray. Chun and her playing partner, Shiho Oyama, of Japan, also opened with birdies on No. 1, and by the turn, each stood at 6 under for the championship. When Yang three-putted the ninth, they, too, were within two shots of the lead.

World No. 1 Inbee Park also made a late push until an uncharacteristic three-putt bogey at the 17th ended her momentum. A birdie on 18 pushed Park into a tie for third with Lewis at 5-under 275 with a final-round 67.

The seemingly unflappable Yang showed signs of unraveling midway through the inward nine. When she bogeyed 14 there was suddenly a three-way tie among Yang, Lewis and Chun at 6 under with four holes to play. At 15, Chun holed a birdie putt to take the lead, while Yang bogeyed and Lewis took her second double-bogey 6 of the round.

“Walking to the 16th tee, she was so relaxed and comfortable,” said Herden, who landed on Chun’s bag through good friend Hee Kyung Seo, the runner-up in the 2011 Women’s Open to Ryu. “We felt like we were leading [after making birdie] at 15.”

With the tee moved up more than 100 yards at the par-4 16th to 235 yards, Chun drove her ball into a greenside bunker. Her 10-foot birdie briefly gave Chun a three-shot lead. Yang responded by holing a 12-footer for eagle on 16 and followed with a 7-foot birdie at the par-3 17th, a hole that Chun also birdied after stuffing her tee shot to 4 feet.

At 18, Chun pulled her tee shot into heavy rough left of the fairway, forcing her to lay up well short of the green. Her 9-iron third shot stopped 12 feet right of the flagstick and she just missed the par putt, giving Yang one final opportunity to tie or take the lead. But she also drove her ball into the left rough and couldn’t reach the green with a hybrid from a difficult lie. Her third from 62 yards spun back to 10 feet below the hole. With Chun watching from the scoring area, Yang’s putt drifted slightly left.

“I hit a good putt,” said Yang. “I aimed about a cup to the left side and all I could think was just [don’t] leave it short.”

Added a gracious Lewis, who posted a second consecutive top-3 finish in the Women’s Open: “I think anytime you have a championship like this, you want to have somebody go out there and win it, and that's definitely what happened there at the end. I was pulling for Amy. I wanted to see a playoff. I wanted to see them come up 18 again with all the crowds there. That's what you want. For both of them to birdie 17 is pretty unbelievable for me. They both played some unbelievable golf.”

With this being Chun’s first USGA championship, she arrived early to get acclimated to the conditions and the area. Being here during July 4 weekend, she also learned a little bit of American history.

Then she went out and made some of her own.

David Shefter is a senior staff writer for the USGA. Email him at dshefter@usga.org.