Venus Williams and Serena Williams at the 2016 Olympic Games in Rio de Janeiro on Aug. 7, 2016
MARTIN BERNETTI/AFP/Getty Images

In a shocking Olympic upset, the Williams sisters, Serena and Venus, lost an Olympic match as partners for the first time in the opening round of the Rio de Janeiro Games against the Czech Republic, the Associated Press reports.

As the report notes, the duo entered Sunday's match with a 15-0 career record at the Olympics, securing three gold medals in women's doubles when they played together in 2000, 2008 and 2012. The sisters were seeded No. 1 in Rio and were just coming off a 14th grand-slam championship together at Wimbledon.

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However, the Czech Republic's Lucie Safarova and Barbora Strycova—neither having ever won any sort of match as a pair, let alone at the Olympics, and who were unseeded in Rio—snatched Sunday's win from the Williams sisters, 6-3, 6-4.

"We played terrible," Serena Williams said, according to ABC News, "and it showed in the results."

According to AP, Safarova and Strycova were not even meant to be playing together at the Olympics; Strycova was a late replacement for Karolina Pliskova, who withdrew from the tournament. The newly formed duo had played only one match together before Sunday night's match and had lost that in a Fed Cup match last year.

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AP notes that the doubles match was Serena Williams' second match of the day after winning her first-rounder in singles earlier. Venus Williams, however, was beaten in her singles first-rounder Saturday.

After that loss, Venus Williams did not meet with reporters, but the women's Olympic tennis coach, Mary Joe Fernandez, said that Venus Williams had been sick since before arriving in Brazil and had been battling cramping, dehydration and an upset stomach after Saturday's defeat.

Read more at ESPN and ABC News