Second-seeded Jelena Ostapenko was dumped out of the Korea Open in the first round by local wild card Back Da-yeon 3-6, 6-1, 7-6 (4) on Tuesday.
In her third tour-level tournament appearance, all of which have been at the Korea Open, the South Korean player had never advanced past the first round.
It looked that trend was set to continue but Back rallied from 5-2 down in the third set, saving one match point, and then claimed the deciding tiebreaker for the biggest win of her career.
Fifth-seeded Sofia Kenin was also beaten by a home favorite, with 162nd-ranked Jang Su-jeong winning 6-1, 6-4. Jang next plays American Emina Bektas, who beat Laura Pigossi 6-7 (5), 6-4, 7-5 in a three-hour duel.
Fourth-ranked Jessica Pegula won her opening round match beating No. 74 Viktoria Hruncakova 6-2, 6-4.
Waiting for the top-seeded American in the next round is compatriot Ashlyn Krueger, who earlier beat Nadia Podoroska 6-2, 6-4.
Also, Eva Lys beat Anna Karolina Schmiedlova 6-2, 6-4, fourth-seeded Marie Bouzkova downed Kayla Day 6-4, 6-4 and Kimberly Birrell defeated qualifier Sachia Vickery 6-4, 6-4.