RIO DE JANEIRO — Serena and Venus Williams never had lost an Olympic doubles match until Sunday night, going 15-0 and earning three gold medals together.

Their first-round opponents at the Rio de Janeiro Games, the Czech Republic’s Lucie Safarova and Barbora Strycova, never had won any sort of match as a pair, at any event. And they had zero wins between them in Olympic doubles competition, going a combined 0-3 with other partners.

So it was rather stunning, to say the least, when the Williams sisters were beaten 6-3, 6- 4 by Safarova and Strycova in the first round in Rio.

“We played terrible,” Serena said, “and it showed in the results.”

The American duo was seeded No. 1 and coming off a 14th Grand Slam championship together at Wimbledon a month ago.

The Czechs, meanwhile, are unseeded. And get this: They weren’t even supposed to be playing together at the Olympics. Strycova was a late replacement for Karolina Pliskova, who withdrew from the tournament.

Indeed, Strycova and Safarova only had played one match as a team before Sunday – and they lost that, in a Fed Cup match last year.

“That’s true, but we are really good friends. We know each other’s games,” said Strycova, never better than a doubles semifinalist at a Grand Slam tournament. “I know what I have to do on the court, and she knows what she has to do.”

Safarova is a strong doubles player, and she won two major titles in 2015 with Bethanie Mattek-Sands of the United States.

When the Czechs found out who they’d be facing to start things off in Brazil, Safarova said their reaction was: “The draw could be better.”

And then she and Strycova started laughing.

“But it was a challenge,” Safarova continued, “and we love challenges. We had nothing to lose. We stepped out there today and played a great game and deserved to win.”

This was the 34-year-old Serena’s second match of the day: The 22-time major singles champion won her first-rounder in that event earlier Sunday. The 36-year-old Venus, meanwhile, now has two first-round exits at the Rio Olympics, because she was beaten in singles on Saturday night.

After that defeat, Venus did not meet with reporters, but U.S. women’s Olympic tennis coach Mary Joe Fernandez said that the seven-time major singles champion had been sick since before she arrived in Brazil. Fernandez also said Venus was dealing with cramping, dehydration and an upset stomach after Saturday’s loss.

In the doubles, played under the lights, when occasional chants of “USA!” would rise from some spectators, other members of the crowd would respond with boos.

Safarova and Strycova did their best to keep hitting shots toward Venus when she was at the baseline, while Serena was left to stand near the net and watch those exchanges.

“I wasn’t playing the way I needed to play,” Serena said. “I wasn’t crossing the way I need to cross.”

The Williams sisters appeared to be getting back into the match in the second set, which was even at 4-all.

But Venus got broken there, allowing Strycova to serve out the surprising victory.

“Lucie was playing unbelievable from the baseline,” Strycova said. “She was hitting amazing winners, returns.”

At that point, Safarova interjected: “And Barbora was a magician at the net.”