Mai Mihara came from fourth place in the women's short program to win the Four Continents Championships on Saturday.
The 17-year-old Mihara, making her debut in the competition, topped the free skate and claimed victory with a total of 200.85 points at Gangneung Ice Arena, almost 10 points more than her personal best.
Short program leader Gabrielle Daleman of Canada placed second with 196.91 points and American Mirai Nagasu was third with 194.95.
"I can't believe I have finished in first place. It's like a dream," said Mihara, who nailed all of her jumps in an error-free performance.
"I had a lot of support and that really provided me with power."
Wakaba Higuchi (172.05) finished ninth and Rika Hongo (1647.42) was 10th.
"I don't think I have ever performed that badly," Higuchi said. "I want to make use of this experience and produce a convincing, error-free performance at the world championships."
Hongo was an injury replacement for Satoko Miyahara, last year's winner who is sidelined wiht a hip injury.
"I think I have to reflect on the glaring mistakes I made on my jumps," said Hongo. "I'll try and perform better at the Asian Winter Games (starting in Sapporo on Sunday)."
In the pairs, Sumire Suto and Francis Boudreau-Audet placed 10th and Miu Suzaki and Ryuichi Kihara finished 13th. China's Sui Wenjing and Han Cong claimed the title with 225.03 points.
Olympic champion Yuzuru Hanyu is third behind American national champion Nathan Chen and Shoma Uno heaeding into the men's free skate on Sunday.