Wildcard Dayeon Back (21), down 5-2 in the third set, rallied to beat second-seeded Jelena Ostapenko (26) 3-6, 6-1, 7-6 (4) in two hours, 13 minutes to advance. Su-Jeong Jang (28) ousted Sofia Kenin (24), the fifth seed, 6-1, 6-4 in 91 minutes.
It was the first time two South Koreans won their opening matches in Seoul since 2013.
For Jang, ranked 162 in the world, the win over Kenin was her first career victory over a Top 30 opponent.
Back, ranked 569, was able to force Ostapenko into long rallies, and the mistakes piled up for Ostapenko. She finished with 68 unforced errors compared to 43 winners.
Top-seed Jessica Pegula (29) avoided the upset bug, beating Viktoria Hruncakova (25) of Slovakia 6-2, 6-4. Number four seed Marie Bouzkova (25) of the Czech Republic also advanced in the WTA 250 event.
Zhengzhou Open
Vera Zvonareva (39) achieved her first Top 20 win since 2021 with a 6-2, 7-6 victory over ninth-seeded Veronika Kudermetova (26) in China.
Zvonareva, the one-time world number two, is still working her way back from a foot injury that kept her out of action from March 2022 until February.
In other first-round action, Liudmila Samsonova (24) 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 (35).
Also advancing at the WTA 500 tournament were number eight seed Daria Kasatkina (26) of Russia, Petra Martic (32) of Croatia, Laura Siegemund (35) of Germany and Zhuoxuan Bai (21) of China.
Hong Kong Open
Canadian Leylah Fernandez (21) advanced to the second round when top-seed Victoria Azarenka (34) retired.
Their match was knotted at one set each when the Belarussian called it a day after appearing to hurt her left leg. Fernandez took a 2-6, 6-3 walkover win.
Also advancing at the WTA 250 event were fifth seed Anna Blinkova (25) and fellow Russians Anastasia Pavlyuchenkova (32) and Mirra Andreeva (16) of Russia, as well as number six seed Martina Trevisan (29) of Italy.
Another Russian, Kamilla Rakhimova (22), beat eighth seed Varvara Gracheva (23) of France 6-3, 6-2, and Czech Linda Fruhvirtova (18) topped seventh-seeded Peyton Stearns (22) in three sets.
