Katie Ledecky shrugged off disappointment in the 200m freestyle on Wednesday by winning the gruelling 1500m just 75 minutes later, as the American finally claimed gold in Tokyo.

Ledecky succumbed again to Australian Ariarne Titmus in the 200m free yet recovered with a dominant swim in the mile, the 24-year-old completing a remarkable feat of physical and mental endurance at the Tokyo Aquatics Centre.

"I definitely wanted to get at least one gold here so I can finally check that box," said Ledecky.

After being pipped by Titmus to gold in the 400m free on Monday, Ledecky was always second-favourite to the Australian in the shorter 200m.

But Ledecky failed even to make the podium, a fifth-place finish her worst in an Olympic final, and she admitted it was difficult to recover.

"It was hard. I thought the better my 200 was, the better my mile was going to be, just from the adrenaline that I got from that," said Ledecky.

"Things didn't work out super-well there but my coach Greg (Meehan) just helped me try to use it to my advantage.

"We were hoping I would have an award ceremony to go to and we would be stressed about getting the warm-down so we just said, 'Hey we have a good chunk of 45 minutes to swim down and get my mind right'.

"I felt I was able to do that and still felt confident going into the mile."

It was an American one-two in the first women's 1500m freestyle at an Olympics as Ledecky clocked 15mins 37.34secs, ahead of teammate Erica Sullivan (15:41.41) and Germany's Sarah Kohler (15:42.91).

'Close to tears'

Ledecky was visibly emotional at the end as she embraced Sullivan over the rope before screaming towards her American support in the stands. She was on the brink of tears as she left the pool.

"I was close (to tears)," she said. "Just after the 200 I knew I had to turn the page very quickly. In the warm-down pool I was just thinking of my family and every stroke I was thinking of my grandparents.

"They're the toughest four people I know. That's what helped me get through that.

"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.

"I'm just proud to bring home a gold medal for Team USA."

After winning four gold medals and a silver at the Rio Olympics, Ledecky has now collected one gold and a silver in Tokyo, with the 800m and potentially a relay still to go.

Ledecky was asked how she has coped with the pressure in Japan, after American gymnast Simone Biles withdrew from the women's team final on Tuesday, citing mental health issues.

"I feel like I handle the pressure," said Ledecky. "The biggest pressure I have is the pressure I put on myself and I think I've gotten past that over the years."

Ledecky's 1500m time was well outside her own world record 15:20.48 but it was still one of the fastest ever.

Yet Sullivan went close, with a powerful final 200m narrowing the gap, even if she said there was no chance of catching Ledecky.

"Catch her, no! Do you know who that girl is? Hell no," Sullivan said. "I didn't realise I got second... I was like, 'Oh wow, Katie must have done something really good', and then I looked up and I was like, 'Oh shoot, I did something really good!'"

The women's 1500m is one of three new events on the Olympic swimming programme this year, along with the men's 800m free, and the mixed 4x100m medley relay.