In an unforgettable display of Olympic power Michael Phelps won two more finals to take his historic all-time record load to 21 golds and entrench his legendary status.

His majesterial display came on a day that Hungary's 'Iron Lady' Katinka Hosszu took her third gold of the week while American gymnast Simone Biles made a flying start to her bid for a record five Rio Olympic golds. But Serena Williams crashed out of what could be her final Games.

Phelps beat Japan's Masato Sakai by just four hundredths of a second to take the 200 metres butterfly and just over an hour later anchored the US 4x200m freestyle relay team to victory.

At 31 -- the oldest individual Olympic swimming gold medallist ever -- Phelps is still the master of the pool.

After claiming his 20th title in five Olympics -- beating old rival Chad le Clos in the process -- Phelps stood in the pool striking a pose like a Roman emperor as he soaked up the acclaim.

After an emotional medals ceremony, he climbed across the massed ranks of photographers to kiss fiance Nicole Johnson and their child Boomer.

It was another dramatic night in the pool with Hosszu completing the individual medley double by winning the 200m final in an Olympic best 2:06.58.

Katie Ledecky, leading the new generation of American swimming stars, captured her second gold of the Games by holding off a brave charge from Sweden's Sarah Sjostrom to take the women's 200m freestyle final in 1:53.73.

The United States is also looking forward to a new era of gymnastics domination with Biles, tipped as the biggest thing since Nadia Comaneci, helping the USA women's team to victory by eight points over Russia.

She is aiming for an unprecedented five titles this week.

"It's everything and more than I'd hoped it to be," said the tiny 19-year-old Texan who has set gymnastics alight with 10 world titles in the past three years.

As her Olympic odyssey got underway, another crumbled as defending tennis champion Williams suffered a shock 6-4, 6-3 defeat to Ukraine's Elina Svitolina in the third round.

Williams, hampered by a shoulder injury, served five double faults in one game of the second set as her own bid to reach five gold medals ended.

The 34-year-old and her sister Venus were earlier knocked out of the women's doubles, the three-time champions' first ever defeat together in Olympic competition.

"It was a great opportunity. It didn't work out the way I wanted it to, but at least I was able to make it to Rio. That was one of my goals," she said.

French Open champion Garbine Muguruza also crashed out, losing 6-1, 6-1 to Monica Puig of Puerto Rico.

Green water

In diving, competitors were perplexed to find the water had turned from light blue to green overnight, although organisers insisted it was safe.

China's Chen Ruolin shrugged off the colour-change to claim a record-equalling fifth gold medal in the women's synchronised 10m platform, with partner Liu Huixia.

New Zealand's rugby hopes took a blow when superstar Sonny Bill Williams suffered a tournament-ending injury in their shock 14-12 opening defeat to Japan.

France won their first gold of Rio in the equestrian team eventing. French hero Astier Nicolas also took a silver in the individual event behind Germany's Michael Jung.

Chinese weightlifter Deng Wei won the women's 63kg category with a world record after Taiwanese rival Lin Tzu-chi pulled out amid reports of a failed dope test.

Fears over Olympic security meanwhile flared anew after a bus carrying accredited press was attacked in Rio late Tuesday.

It remained unclear whether gunfire or stones were used in the media bus attack, which smashed windows and slightly injured two journalists, according to Rio organisers.

The incident comes after a stray bullet ripped through a media tent at the equestrian venue on Saturday and a controlled explosion near the finish of the men's cycle road race.