Sunday's final will pit the Thai star against third seed Wang Yihan of China, who defeated top-seeded compatriot Wang Shixian 21-19, 21-12 in the other semi-final. Ratchanok, seeded second in the tournament, has never defeated Wang Yihan.
It will be first time in the 23-year history of the Korean Open that the event has not had a Korean finalist.
"I knew that I had to take risks in this match whenever I had the opportunity but when I didn't have opportunities, I basically just tried to keep moving her around the court," said Ratchanok, who outlasted Sung 24-22, 11-21, 21-17.
"She is very tall, so I thought that tiring her out would give me the best chance."
Sunday's match promises to be just as big a challenge for the 18-year-old Thai star known as "Mae" to family, friends and fans.
"The final is going to be tough because I’m playing against the best in China," she told Badzine before the other semi-final. "I have only beaten Wang Shixian once and I have never beaten Wang Yihan so no matter who I have to play, it's going to be a very difficult match."
Ratchanok last August became the youngest-ever world badminton champion at the age of 18, and the first Thai to win a world title in the sport, when she defeated Olympic champion Li Xuerui of China.