But in just over 24 hours, the previously unknown 20-year-old playing in her first professional tournament outside Japan, had charmed the galleries with her charisma and an impressive final score of 18 under par, despite a double-bogey on the third.
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An, who had shared or held the solo lead since Thursday, is searching for his first PGA Tour win and he was able to keep his bogey-free streak alive through 54 holes at the Sedgefield Country Club.
Salas looked set to complete a brilliant 64 following a superb approach to the last only to nervously miss her birdie putt from close range and Shibuno made no such mistake to seal a remarkable victory.
Just a year ago Shibuno, 20, was competing on the Step-Up Tour, a developmental circuit for the Japan LPGA. The two players combined to shoot eight under on the final nine holes, which were particularly kind to Shibuno-she shot even par on the front nine over the four days and 18 under on the back.
Pressel won a major - the 2007 Kraft Nabisco Championship - at the age of 18 but hasn't won on the LPGA Tour since 2008.
"I'm definitely exceeding my expectations", she said.
Others at 10 under are South Korea's Jin Young Ko (68) and Lizette Salas (70).
Park is seeking her third major title, while Ko is looking to become one of a handful of golfers to win three majors in the same year.
Defending Champion Georgia Hall got off to a good start with a pair of 69s but struggled over the weekend with a 74 and 73 to finish tied for 35th, while home favourite Charley Hull dropped back to a share of 24th thanks to a final-round 76.
"I'm so overwhelmed by the amount of support I'm getting" said the world number thirty after her round. "I want to stay aggressive until the end and get a lot of birdies".
"I have so many people out here supporting me and I am pleased I could play well for them".
Eighteen-year-old amateur Yuka Yasuda is at 3-under, followed by Minami Katsu and Ayako Uehara at 2-under.
"I came to the British Open thinking it would be a links course".
"I'm not going to lie, I was nervous", she said.