Aryna Sabalenka was swatted aside in straight sets in the third round of the China Open on Sunday, leaving the women's draw without its top two seeds, while Daniil Medvedev went through for the men.
Czech player Nikola Bartunkova, 20, sent down four aces to beat the former world number one 6-4, 6-3 in one hour and 35 minutes.
"I focused on myself, to be calm," 34th-ranked Bartunkova said.
"She's a legend... so I'm really happy that I won the match," she said of the four-time Grand Slam champion.
Bartunkova next faces Austria's Sinja Kraus in the round of 16.
Second-seeded Sabalenka squandered five chances to break serve in the sixth game of the first set, with Bartunkova strong on defence and firing deep.
The 28-year-old from Belarus threw on a cap for the eighth game of the first set as the women switched between playing in the shade and the sun, but it did not turn the tide in her favour.
A frustrated Sabalenka tossed her racquet at a break down in the fourth game of the second set, and lost on the first match point when she returned into the net.
Top seed Elena Rybakina -- who replaced Sabalenka as world number one weeks ago -- is also out, having lost to Alina Charaeva in the second round on Saturday.
Also on Sunday, French Open champion Mirra Andreeva of Russia set up a fourth-round match with Ukraine's Daria Snigur, while Wimbledon champion Linda Noskova breezed past Viktorija Golubic 6-2, 6-1.
Noskova next faces 28th-ranked Russian player Ekaterina Alexandrova.
In the men's draw, former world number one Daniil Medvedev dispatched Francisco Cerundolo 7-6 (7/4), 6-3 to set up a semi-final thriller with either Alexander Zverev or Novak Djokovic.
Poland's Hubert Hurkacz also advanced to the semi-finals after beating Karen Khachanov, and will play Australia's Alex de Minaur who survived a gruelling test to beat Andrey Rublev 1-6, 6-2, 6-3.
Top seed Zverev plays Djokovic later on Sunday in the quarter-finals.
The 39-year-old Serbian great enters the match with an extraordinary unbeaten streak in Beijing, but Zverev -- last month's US Open champion -- is in the running for the top men's ranking.