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.