Seoul - Ekaterina Alexandrova broke Jelena Ostapenko's serve five times on Friday and defeated the defending champion and No 1-seeded player 6-3, 6-2 in a rain-delayed match at the Korea Open.
Ostapenko, the 2017 French Open champion, had reached the quarter-finals or better at six tournaments this year, including the Wimbledon semi-finals.
Alexandrova came into the match with a 1-7 career record in round-of-16 matches at WTA-level events.
The postponement from Thursday meant Alexandrova had to play her quarter-final match later Friday against sixth-seeded Hsieh Su-wei.
Hsieh is on a seven-match winning streak after capturing last week's Japan Women's Open at Hiroshima.