By Mark Trevelyan
RIO DE JANEIRO (Reuters) - Katie Ledecky of the United States broke her own world record on Sunday on the way to winning the women's 400 metres freestyle gold medal, the first stage of a rare treble she hopes to complete at the Rio Olympics.
Ledecky, who had come close to the record in the morning heats, led from start to finish to set a new mark of three minutes, 56.46 seconds, shaving 1.91 seconds off the previous best she set in Australia two years ago.
"It feels really good. I'm pumped. I was so close to breaking it (this morning)," Ledecky said. "I felt good throughout. The swim was almost identical to this morning, but with a bit more pop on the last lap. It's pure happiness."
It was the sixth world record to fall in just two days of competition at the Olympic Aquatics Stadium.
Jazz Carlin took the silver medal on a great night for British swimmers, minutes after Adam Peaty had won gold in the 100 metres breaststroke.
Leah Smith of the United States won bronze.
Ledecky's winning margin of 4.77 seconds underlined her total dominance of the event.
The 19-year-old is also overwhelming favourite to take the 800 metres freestyle, in which she won gold in London four years ago, and is also competing in the 200m freestyle.
If she wins all three, Ledecky - who also won a silver in the 4x100 freestyle relay on Saturday - will be the first women to achieve the feat since American Debbie Meyer at the Mexico City Olympics in 1968.
The 2012 London champion Camille Muffat of France was among 10 people killed in March 2015 when two helicopters collided in a remote region of Argentina during the filming of a reality TV show.