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World number one Novak Djokovic became the latest big name to crash out of the Tokyo Olympics on Friday as a defeated American swimmer aimed doping accusations at a Russian rival, drawing a furious response from Moscow.
As the athletics began in an Olympic Stadium deprived of spectators by anti-coronavirus measures, Ethiopia's Selemon Barega timed his run to perfection to win the first track gold in the men's 10,000 metres.
But Djokovic's bid for a calendar Golden Grand Slam -- winning all four Grand Slam tournaments plus the Olympics -- was dramatically ended by Alexander Zverev at the Ariake Tennis Park.
The Serb collapsed from a set and a break ahead as German fourth seed Zverev won 1-6, 6-3, 6-1 and will meet Russian Karen Khachanov in the final.
Djokovic's second chance of a gold medal vanished hours later, as he and Nina Stojanovic lost their mixed doubles semi-final to Russian pair Aslan Karatsev and Elena Vesnina 7-6 (7/4), 7-5.
He joins leading players including Naomi Osaka and Ash Barty in failing to make it through the tennis competition.
France's judo superstar Teddy Riner came up short in his bid to win a historic third consecutive heavyweight title, losing to Russia's Tamerlan Bashaev in the quarter-final. Riner had to settle for bronze.
- Doping 'broken record' -
In a bitter row at the pool, American swimmer Ryan Murphy said he had been "swimming in a race that's probably not clean" after Russian Evgeny Rylov beat him in the 200m backstroke.
Rylov said he was "surprised" by Murphy's "strange" suggestion.
The Russian Olympic Committee (ROC) hit back, tweeting "the broken record is once again playing the song about Russia doping and someone is diligently pressing the button on the English-language propaganda".
The Russian Anti-Doping Agency said Rylov had been tested three times this year and that he was "prepared and clean".
Russia are banned from Tokyo 2020 after being found guilty of state-sponsored doping, meaning their athletes cannot use the Russian flag and anthem.
But more than 300 Russian competitors have been allowed to compete under the ROC moniker, and they have won 10 golds to lie fourth in the medals table.
Jamaican sprint queen Shelly-Ann Fraser-Pryce made an impressive start in the 100m as she attempts to become the first woman to win an individual Olympic athletics event three times.
The Jamaican, champion in 2008 and 2012, accelerated out of her blocks and strode over the line for a first-round victory in 10.84sec.
One of her rivals, the Ivory Coast's Marie-Josee Ta Lou, blasted to 10.78sec and reigning champion Elaine Thompson-Herah of Jamaica recorded 10.82sec on what looks to be a fast track.
The semi-finals and final of that event take place on Saturday.
World record-holder Karsten Warholm of Norway strolled through his heat of the 400m hurdles heat, an event that could be one of the highlights.
Qatar's Abderrahman Samba eased through but said he felt the absence of spectators: "It was really, really difficult. I really missed the crowd."
South Africa's Tatjana Schoenmaker was overjoyed after becoming the first South African woman to win an Olympic swimming gold for 25 years as she smashed the world record in the 200m breaststroke, timing 2min 18.95sec.
"It's still so unreal," she said. "I think my emotions in the pool showed I was pretty shocked."
Australia's Emma McKeon claimed her fourth medal in Tokyo as she blazed to the women's 100m freestyle title in a new Olympic record of 51.96sec.
Megan Rapinoe scored the crucial spot kick as the US women's football team stayed alive in a penalty shootout victory over the Netherlands after the match ended 2-2 following extra time. The Americans play Canada in the semi-finals.
Australia beat Great Britain 4-3 after extra time and will face Sweden in the other semi.
- Australians test negative -
The Australian athletics team said three of its members would remain isolated from the rest of the squad "as a precautionary measure" after testing negative for coronavirus.
The trio are classed as close contacts of US pole vaulter Sam Kendricks, who has been ruled out of the Games after testing positive for the virus.
Coronavirus cases are surging in Japan a week into the Games.
On Friday, Tokyo 2020 organisers reported 27 new cases related to the event -- the highest daily figure yet -- although they insist there is nothing to suggest a link between the Games and rising infections in Japan.