An electric Ariarne Titmus dethroned Katie Ledecky to become the 200m freestyle champion Wednesday, only for the gutsy American great to bounce back and clinch the first-ever women's Olympic 1500m freestyle title.

Australian Titmus now has two Tokyo golds, both at Ledecky's expense after upsetting her arch-rival to win the 400m free on Monday.

But the chances of US superstar Caeleb Dressel winning a much-touted seven gold medals in Tokyo vanished when he opted out of the 4x200m relay squad that finished fourth behind winner Great Britain.

Titmus swam the second fastest 200 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.

Australia's Ariarne Titmus added the 200m to her 400m crown
Australia's Ariarne Titmus added the 200m to her 400m crown AFP / Odd ANDERSEN

She powered through the field from third at 150m to touch in a new Olympic record time of 1min 53.50sec, with a sluggish Ledecky fifth.

Hong Kong's Siobhan Haughey took silver (1:53.92), with Canada's Penny Oleksiak third (1:54.70).

"Bloody exhausted, that was 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."

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

Katie Ledecky won the inaugural women's 1500m gold medal
Katie Ledecky won the inaugural women's 1500m gold medal AFP / Oli SCARFF

Despite the crushing loss of two titles, Ledecky was back in the pool barely 75 minutes later for a brutal 1500m, one of three new swim events on the programme this year.

The world record-holder is dominant in the event and didn't disappoint, producing a commanding swim to claim gold in 15:37.34.

Teammate Erica Sullivan (15:41.41) was second and Germany's Sarah Kohler (15:42.91) third.

"I'm so happy to go one-two there with Erica in the first women's mile. You can't have a better outcome than that, I'm so happy," said Ledecky.

Great Britain won the men's 4x200m freestyle relay
Great Britain won the men's 4x200m freestyle relay AFP / Oli SCARFF

"It means a lot. I think people maybe feel bad for me because I'm not winning everything and whatever but I want people to be more concerned about other things in the world, people who are truly suffering."

Ledecky, who won four gold and a silver at the Rio Olympics, has now claimed one gold and a silver in Tokyo with the 800m to go and potentially a relay.

Dressel was absent when the US 4x200m relay team lined up for the final, and they missed him, finishing outside the medals as Britain blitzed to victory.

Led off by Tom Dean and brought home by Duncan Scott, they touched in 6:58.58sec, just outside the 6:58.55 world record held by the United States. Russia were second and Australia third.

Dressel has already won one gold in the 4x100m relay and could feature in two more relays along with his three individual events -- the 50m and 100m freestyle and the 100m butterfly.

Before the relay he clocked 47.23sec to breeze into the 100m freestyle final, marginally slower than Russian dangerman Kliment Kolesnikov (47.11). Australian reigning champion Kyle Chalmers was sixth.

Elsewhere, Hungarian Kristof Milak powered to the Olympic men's 200m butterfly gold medal.

Milak, who crushed Michael Phelps' world record in 2019, hit the wall in a new Olympic record of 1:51.25, more than two seconds clear of Japan's Tomoru Honda and Italy's Federico Burdisso.

But he said that his trunks tore before the race, complaining it cost him.

"They split 10 minutes before I entered the pool and in that moment I knew the world record was gone," he told reporters after the race, angrily throwing the torn togs at a table. "I lost my focus and knew I couldn't do it."

Japan's Yui Ohashi motored to the women's 200m individual medley title, making it a Tokyo double after her earlier triumph in the 400m medley.

She produced a strong freestyle leg to touch in 2:08.52 in front of Alex Walsh and fellow American Kate Douglass, with defending champion Katinka Hosszu seventh.

In semi-final action, China's Zhang Yufei was dominant in the women's 200m butterfly, while Australia's Zac Stubblety-Cook led the field into the men's 200m breaststroke decider.