A female athlete from the favelas won Brazil's first gold medal of the Rio Games on Monday as Michael Phelps stepped up his campaign for yet more Olympic honours.

Rafaela Silva, who grew up in Rio's notorious City of God slum, upset world number one Sumiya Dorjsuren to win the women's under-57kg judo, sparking pandemonium among fans.

Silva sank to her knees in delight after a win that jump-starts Brazil's Olympic campaign and secures only their second medal after Felipe Wu's shooting silver.

Earlier Phelps, who grabbed his 19th Olympic gold medal late on Sunday, was back in action as he eased into the semi-finals of his signature event, the 200m butterfly.

"I probably got to sleep at 3:00 am and was on an 11:00 am bus, so quick turn-arounds," Phelps said. "But the good thing is we have a long time between the prelims and finals, so we are able to rest."

Katie Ledecky and Sarah Sjostrom also had a fast turn-around as they tackled the 200m freestyle heats after world record-breaking wins on Sunday night.

"It was pretty hard coming off of last night," said Ledecky, who was accepting her 400m freestyle gold as the clock struck midnight.

"Just got a couple hours of sleep, but I think that was probably going to be my hardest swim of the week so I'm glad it's over with."

Hungary's 'Iron Lady' Katinka Hosszu will be one to watch when she competes in Monday's 100m backstroke final, after smashing the 400m individual medley world record on Saturday.

Also in focus, but for very different reasons, will be Russia's Yuliya Efimova who has been taunted by fans and fellow swimmers over her drug-tainted past.

Red flag

Officials sought to calm trash-talking between athletes after Australia's Mack Horton called China's Sun Yang a "drugs cheat", sparking a furious response from Chinese state media.

"Clearly we want to encourage freedom of speech," said International Olympic Committee spokesman Mark Adams.

"But on the other hand of course the Olympics is about respecting others and respecting the right of others to compete," he added.

"There is a line somewhere there, and each case is different of course, where people should be free to compete in tranquility.

Organisers also scrambled to replace the Chinese flag used at official ceremonies after the discovery of a flawed design drew an official complaint.

"We do understand that there is a problem with the flag. It's very small," said Games spokesman Mario Andrada.

"You have to be very familiar with the Chinese flag to understand that. However we need to correct it."

The four small, gold stars on the Chinese flag are pointing upwards rather than towards the bigger star, as they are supposed to, a mistake which was poorly received by patriotic Chinese.

However, Chen Aisen and Lin Yue won the men's 10m platform diving, putting China top of the medals table with four golds midway through day three.

Russia took gold and silver in the women's sabre fencing as Yana Egorian beat Sofiya Velikaya, who again suffered heartbreak after also losing the 2012 Olympic final.

And Thailand also enjoyed a one-two in the women's 58kg weightlifting as Sukanya Srisurat beat Pimsiri Sirikaew by 8kg.

It comes after the 84-year-old grandmother of Thailand's Sinphet Kruaithong collapsed and died while celebrating his bronze medal in the 56kg category on Sunday.

In rugby, Australia and New Zealand will vie to become the first Olympic champions in 92 years when they clash in the women's final later Monday.