'Ageing' Bolt takes second Rio gold

Rio De Janeiro, Usain Bolt won his second gold medal of the Rio Olympics, brushing aside all-comers again in his favourite 200m sprint but failing in his bid to smash his own world record.

The Jamaican triumphed as Britain's Brownlee brothers dominated in triathlon to keep their country ahead of China in the medals race.

Meanwhile, an Iranian woman finally won an Olympic medal.

The 29-year Bolt timed 19.78sec to easily beat Canadian Andre de Grasse into second and France's Christophe Lemaitre into third, both of whom failed to get under 20sec.

Before the race Bolt had said he would go for his world record of 19.19sec.

Bolt said it was "a brilliant feeling" to win but he was not happy with his time and indicated he may not run the 200m at the world championships next year in London.

"I wasn't happy with the time, my body just wouldn't respond in the straight. I'm getting older and my body is ageing.

"Personally I think this is my last 200 but my coach may beg to differ."

He also insisted he belonged in the pantheon of all sports legends.

"I am trying to be one of the greatest...to be among Ali and Pele. I hope after these Games I will be in that bracket."

A triumph in Friday's 4x100m relay with Jamaica would give him a 'triple triple' -- third straight sweep of the sprint golds -- to bid farewell to the Olympics.

The reigning US 4x100m women's relay champions ran alone on the track to get a place in the final after being given a reprieve by the IAAF.

They were left facing disqualification after a dropped baton exchange between Allyson Felix and English Gardner.

Judges ruled Felix had been knocked off-balance by Brazilian runner Kauiza Venancio and allowed a race against the clock.

The Americans managed it, got the fastest time and forced China out of the final.

Trinidad-born American Kerron Clement won the men's 400m hurdles gold, clocking 47.73sec to hold off a furious late challenge from Kenya's Boniface Tumuti. Turkey's Cuban-born Yasmani Copello claimed bronze.

Dalilah Muhammad became the first US winner in the women's 400m hurdles. The fastest woman in the world this year over the distance, blasted out of the blocks and held on to a commanding lead for a deserved gold in 53.13sec.

Sara Slott Petersen of Denmark took silver while Muhammad's team-mate Ashley Spencer claimed bronze.