Ariarne Titmus admitted she was exhausted, but excited, after winning gold to dethrone American great Katie Ledecky in the Olympic 200m freestyle final on Wednesday, having already taken her 400m crown.

The Australian touched in a new Olympic record time of 1min 53.50sec, with a sluggish Ledecky relegated to fifth. Hong Kong's Siobhan Haughey took the silver (1:53.92), with Canada's Penny Oleksiak third (1:54.70).

Titmus toppled Ledecky as the 400m champion on Monday and again proved too good over the shorter distance, powering through the field to win after turning at 150m in third.

She still has the 800m freestyle and the 4x200m relay to go at the Tokyo Aquatics Centre in a gruelling programme.

"Bloody exhausted, that was a tough one," said Titmus. "Honestly, it's not the time that I thought I could do this morning but it's the Olympics and there's a lot of other things going on. So it's just about winning here and I'm very happy."

Her coach Dean Boxall, whose wild celebrations after her 400m win went viral, celebrated in more muted fashion on Wednesday.

The Australian clocked the second-fastest 200m in history last month (1:53.09) to signal her intentions, ranking only behind Federica Pellegrini’s super-suited world record of 1:52.98 from 2009. Pellegrini finished seventh on Wednesday.

Ledecky touched in 1:55.21, with her tough Tokyo schedule perhaps taking its toll, although she picked herself up to return to the pool and win 1500m gold.

"Obviously having a great swim in the 400 gave me confidence coming into the 200," said Titmus, who was in tears after receiving her medal.

"I thought that my back end was definitely my strength in the 400 so I knew that I could have that on the way home in the 200.

"But every race is different. You might swim well in one and then not be able to pull it out in the next, so I can't rely on that confidence too much."

Titmus, 20, has become the new face of Australian swimming and she is taking it in her stride.

"I feel so fortunate to be here, to be able to do what I love," she said.

"I'm just from a small town in Tasmania and it just goes to show if you believe you can do something you can 100 percent do it if you work for it."

Her parents were on the edge of their seats watching back home, with father Steve admitting his nerves were shredded.

"I nearly had a heart attack," he told Channel Seven television.

"What an incredible comeback. We knew she would come home really, really hard. Wow, that last 25, what a racer, what guts. What determination."