Serena and Venus Williams authored a bit of Olympic history Sunday.

Just not the type they're used to making.

The Williams sisters dropped a 6-3, 6-4 straight set decision to the Czech Republic duo of Lucie Safarova and Barbora Strycova in the first round of women's doubles.

The loss was the Williams' first in women's doubles Olympic competition. They had been 15-0 in Olympic competition, and had won gold in the 2000, 2008 and 2012 Olympics.

Later in the day top-ranked Novak Djokovic lost his first-round Olympic tennis match to 2009 U.S. Open champion Juan Martin del Potro

The Williams sisters entered Sunday's match with a 15-0 career record in the Olympics, winning the gold medal in women's doubles every time they entered the event.

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

Neither Safarova nor Strycova, meanwhile, ever had won an Olympic doubles match, going a combined 0-3 with other partners. They are unseeded in Rio.

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 night — and they lost that, in a Fed Cup match last year.

This was Serena's second match of the day: She won her first-rounder in singles earlier Sunday. 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.

However, U.S. Tennis Association spokesman Tom LaDue told The Associated Press earlier in the day that Williams felt better and was able to compete.

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.

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.

The Williams' committed four double faults and 15 unforced errors in the loss. By comparison, Safarova and Strycova recorded 26 winners.

The loss was the culmination of a day which began with Serena's 6-4, 6-2 victory over Australia's Daria Gavrilova. The match was Williams' first since she won her Open-era record-tying 22nd Grand Slam title at Wimbledon last month.

The American, seeded No. 1, was hardly at her best in the 91-minute contest, compiling 37 unforced errors to 27 winners, and she got broken at love while serving for the first set at 5-3. But Williams broke right back in the next game.

Along with her three gold medals in doubles, the younger Williams won gold in singles at London.