Serena and Venus Williams may be two of the biggest tennis stars in the world, but they got off to a bad start in their Rio Olympics debut as a duo.

The Williams sisters were defeated by Czech Republic's Lucie Safarova and Barbora Strycova in the first round of the women's tennis doubles Olympics tournament on Sunday. The Czech athletes defeated Serena and Venus 6-3, 6-4, dealing a devastating loss for the popular sisters.

After the match, Venus and Serena spoke with the press and suggested that they did not play to their full potential.

"Today was not our best day as a team," Venus admitted. "We like to have a chance to play into the tournament, we just played a tough team. So we wish them the best of luck in the rest of the event, and we'll get ready for the U.S. Open in doubles."

"It's devastating," Serena said. "But we're really, also, I wouldn't say it's devastating. We had a chance to compete for our country, we did the best that we can, and we had a blast out there. I wouldn't actually say it was devastating, it was a lot of fun, and we'll always remember these moments, these matches. At the end of the day, that's what matters most."

Serena candidly admitted that she and her sister "didn't play our best," adding, "We played terrible, and it showed in the results."

Serena and Venus previously had a 15-0 at the Olympics, winning the Gold medal for women's doubles tennis in 2000, 2008 and 2012.

Venus had a particularly bad day considering she also lost her singles match against Belgium's Kirsten Flipkens by a score of 4-6, 6-3, 7-6 (5). Meanwhile, Serena defeated her first round opponent, Australi's Daria Gavrilova by a score of 6-4, 6-2. Serena will be playing France's Alizé Cornet at 5:45 p.m. ET, which you can live stream here.