U.S. swimmer Michael Phelps added to his Olympic record, winning a 19th gold medal on Sunday night, while American Katie Ledecky smashed her own world mark in the 400-meter freestyle at the Summer Games.
In his first race in Rio, Phelps swam a powerful second leg in the 4x100-meter freestyle relay. His 47.12 split gave the Americans a clear lead, which they held for a narrow win over France.
Now in his fifth Olympics, Phelps has won 23 medals overall and still has a shot at more.
Ledecky, 19, arrived in Rio as the most dominant female swimmer in the world, and she delivered in her first individual event. It was a race only in a loose sense of the word, as Ledecky took a big early lead and finished in 3 minutes, 56:46 seconds.
That was nearly two seconds better than her own world record and more than four seconds ahead of the second-place finisher.
The teenager from suburban Washington, D.C., will also be the heavy favorite in two other individual races, the 200- and 800-meter freestyle.
She won a silver medal Saturday as the anchor of the 4x100-meter freestyle relay team, which finished a close second to Australia.
Simone Biles dazzles
In her first performance in Rio, Biles immediately demonstrated why she's been world champion for the past three years.
She sparkled in all four events — floor, vault, balance beam and uneven bars — generating a score of 62.366, far ahead of the second-highest scorer, American teammate Aly Raisman.
Sunday's events were qualifying for finals later this week. The five-woman American squad far outdistanced China and will be a prohibitive favorite on Tuesday to defend the team gold they won in London four years ago.
The American women were quite literally competing against themselves. Only two women from a country can advance to the individual all-around final later this week.
Gabby Douglas, who won the all-around gold in London, turned in a strong performance and was the third-highest scorer Sunday among all gymnasts.
Yet Biles and Raisman claimed the two spots in the individual all-around, and Douglas won't be in the finals to defend her title.
Shocker: Serena and Venus Williams lose
One of the surest bets in Rio seemed to be sisters Serena and Venus Williams in women's doubles. They had won gold three times previously, had a perfect 15-0 record in the Olympics, and were seeded No. 1.
Yet they were bounced in the first round by the Czech Republic's Lucie Safarova and Barbora Strycova, 6-3, 6-4.
The Czechs had only played one match together, and lost that one last year. But they efficiently dispatched the Americans, directing most of their shots toward Venus Williams.
After the match, the U.S. team coach, Mary Joe Fernandez, said Venus Williams was ill when she arrived in Rio and suffered from cramps and dehydration. Venus Williams also lost her first-round singles match on Saturday.
Serena Williams, who's seeded No. 1 in singles, easily won her opening round match earlier in the day Sunday.
New to the games, Kosovo wins gold
Just two days into its very first Olympics, Kosovo has already won a gold medal.
Majlinda Kelmendi won in the 52-kilogram (114-pound) weight class in women's judo on Sunday by defeating Japan's Misato Nakamura in the semifinals and then Italy's Odette Giuffrida in the finals.
While representing a country that's never been to the games, Kelmendi herself was not a first-timer. She was able to compete for neighboring Albania at the 2012 Games, though she did not medal.
As we've noted, Kosovo and South Sudan are both new to the games this year as independent countries and have sent small contingents. Kosovo sent eight athletes, and South Sudan has three runners in Rio.
Kosovo, which fought a war with Serbia in the late 1990s, declared independence in 2008 but was not recognized by the International Olympic Committee until 2014. South Sudan gained independence in 2011 after decades of war with Sudan.
Russian Paralympic team banned
Russia's entire Paralympic team will be barred from next month's Paralympics in Rio de Janeiro in the latest blow to the country over its doping scandal.
"The anti-doping system in Russia is broken, corrupted and entirely compromised," Sir Philip Craven, president of the International Paralympic Committee, told a news conference in Rio on Sunday.
"I believe the Russian government has catastrophically failed its para athletes. Their medals over morals mentality disgusts me. The complete corruption of the anti-doping system is contrary to the rules and strikes at the very heart of the spirit of Paralympic sport," he added.
Russia had 267 slots for its para athletes in 18 sports, and those positions could now go to other countries, the Paralympic Committee said.
The Russians won the second most medals at the 2012 Paralympic Games in London, trailing only China.
The Paralympics will be held in Rio on Sept. 7-18.