Katie Ledecky obliterated the field and her own world record in winning the 800m freestyle at the Rio Olympics on Friday, uniting the 200m, 400m and 800m titles for the first time since 1968.

The 19-year-old American, who also won the 4x200m relay for four golds overall, touched in 8min 04.79sec, beating the previous record of 8:06.68 she set in Austin, Texas, on January 17.

It was her 13th world record since her shock 800m freestyle win in London four years ago, and the culmination of the plan she and coach Bruce Gemmell had worked for ever since that breakout performance.

And as she contemplated her achievement in Rio, the preternaturally poised teen couldn't hold back her emotions.

"Just kind of the end of a four-year journey," she said as the tears flowed.

"I don't know why I'm crying. There were nights I would go to bed and think about this day (and) how much fun I've had these past four years, and I'd start crying in bed.

"I just wanted to make this meet count."

Ledecky touched almost half a length in front of her closest rivals -- more than 11 seconds in front of silver medallist Jazz Carlin of Britain's 8:16.17 with Boglarka Kapas of Hungary taking bronze in 8:16.37.

"I just wanted to lay it all out there," Ledecky said. "It was my last swim here at the Olympics, the pinnacle of our sport, and I have to wait another four years to have that moment. I just wanted to enjoy it all and put my very best swim out there."

Ledecky joined compatriots Janet Evans and Brooke Bennett as the only swimmers to win this event at two Olympics and now owns the top 11 times ever in the event.

Her dominant performance in Rio also included silver in the 4x100m freestyle relay.

Ledecky is the first Olympian to take the 200m, 400m and 800m gold at a single Games since another US teenager, Debbie Meyer, achieved the feat at Mexico City in 1968 -- the year the 800m was added to the programme.

American swimmer Maya Dirado denied Katinka Hosszu a fourth Rio Olympics individual swimming gold medal, pipping the Hungarian in a women's 200m backstroke thriller.

Dirado finally overhauled "Iron Lady" Hosszu after finishing second behind her in the 400m individual medley and third in the 200m individual medley.

Dirado, who won 4x200m freestyle relay gold in Rio, tracked front-runner Hosszu throughout, edging ahead in the final meters to win in 2min 05.99 seconds.

Hosszu took the silver in 2:06.05 and Canada's Hilary Caldwell claimed bronze in 2:07.54.

Hosszu, won the 100m backstroke along with the 200m and 400m medleys, was bidding to equal the record for the most gold medals won by a female swimmer in individual events at a single Olympics.

The only other woman to have achieved the feat was East Germany's Kristin Otto at Seoul in 1988.