World number 34 Nikola Bartunkova eliminated second seed Aryna Sabalenka in straight sets 6-4, 6-3 in the China Open third round in Beijing on Sunday.
The 20-year-old Czech player needed one hour and 35 minutes to secure the biggest victory of her career, recording four aces along the way.
The result means the tournament will proceed without its top two seeds after earlier upsets in the women's draw.
Bartunkova Stays Calm to Upset Four-Time Grand Slam Champion
"I focused on myself, to be calm," Bartunkova said after the match.
She acknowledged the stature of her opponent, a four-time Grand Slam champion, calling her "a legend" and expressing happiness at winning.
Sabalenka missed critical opportunities early in the opening set, squandering five break chances in the sixth game as Bartunkova held firm with deep groundstrokes and strong defensive play.
The 28-year-old Belarusian struggled with variable lighting conditions across the court, donning a cap in the eighth game of the first set before throwing her racquet after falling behind a break in the second set.
The match concluded when Sabalenka hit a return into the net on Bartunkova's first match point, sending the Czech player to a round of 16 matchup against Austria's Sinja Kraus.
Sabalenka's exit followed the departure of top seed Elena Rybakina, who recently claimed the World No. 1 ranking from Sabalenka but lost to Alina Charaeva in the second round on Saturday.
In the men's 500-level tournament held simultaneously in Beijing, top seed Alexander Zverev was set to clash with Novak Djokovic in the quarterfinals later on Sunday, as Djokovic defended an unbeaten streak in Beijing against the reigning US Open champion.
