Ariarne Titmus quest for a third victory against US arch rival Katie Ledecky remains on track after Australia's new golden girl qualified for the final of the 800m freestyle, but admits she is already feeling exhausted by her Olympic schedule.

The Australian, who has already won the  200m and 400m freestyle at these Games, qualified with the sixth fastest time in Thursday night's heats, more than three seconds slower than Ledecky. 

Titmus finished second in her heat to post a time of 8.18.99 behind Russian Anastasiia Kirpichnikova, having backed up hours after being part of the 4x200m relay team that was surprisingly beaten in the final, finishing third.

'It’s a big day today. I had one and a half hours in my room today. I’m absolutely buggered,' Titmus said.

'Honestly I’m exhausted. 

'(US swimming great) Phelps won eight golds (at the 2008 Olympics). I have done two and I’m wrecked. It puts into perspective for me how amazing he was.

'I’m going to rest up as much as I can and get ready for my last race at the Olympics.'

Ledecky posted a slick time of 8.15 minutes to win the next heat and post the quickest time for the final after she anchored the US to silver in the 4x200 behind shock winner China. 

Fellow Aussie Kiah Melverton is also through to the 800m final after finishing third in Titmus' heat.

Titmus has already won two showdowns again her arch rival Ledecky, who holds the world record in the 800m which she won in 2012 and 2016.

'I love racing Katie. It's been really good this week,' Titmus added.

'We've been getting along really well. We've really enjoyed racing each other. We don't get to do it often. It's so fun for us to be able to race. So, I am looking forward to two days time.' 

The final will be held on Saturday morning.