Wild card Dayeon Back, down 5-2 in the third set, rallied to beat second-seeded Jelena Ostapenko 3-6, 6-1, 7-6 (4) in two hours, 13 minutes to advance.
It was the first time two South Koreans won their opening matches in Seoul since 2013.
569, was able to force Ostapenko into long rallies, and the mistakes piled up for Ostapenko.
In other first-round action, Liudmila Samsonova of Russia, fresh off her finals appearance in the China Open, continued her strong play with a 6-3, 6-3 defeat of Germany's Tatjana Maria.
Another Russian, Kamilla Rakhimova, beat eighth seed Varvara Gracheva of France 6-3, 6-2, and Czech Linda Fruhvirtova topped seventh-seeded Peyton Stearns in three sets.
Persons:
Jelena Ostapenko, Russia's Liudmila Samsonova, Florence, Jeong Jang, Sofia Kenin, Jang, Kenin, Jessica Pegula, Viktoria, Marie Bouzkova, Vera Zvonareva, Veronika Kudermetova, Zvonareva, Germany's Tatjana Maria, Daria Kasatkina of, Petra Martic, Laura Siegemund, Zhuoxuan Bai, Canadian Leylah Fernandez, Victoria, Fernandez, Anna Blinkova, Anastasia Pavlyuchenkova, Martina Trevisan, Kamilla, Varvara Gracheva, Czech Linda Fruhvirtova, Peyton Stearns
Organizations:
National Tennis Center, Hana Bank, South Koreans, Ostapenko, Viktoria Hruncakova, WTA, China . Hong Kong, Victoria Azarenka, Thomson
Locations:
Beijing, China, Hana Bank Korea, Seoul, Slovakia, Czech Republic, Zhengzhou, Russia, Daria Kasatkina of Russia, Croatia, Germany, China . Hong, Canadian, Italy, France, Czech