Katie Ledecky has exacted revenge on arch-rival Ariarne Titmus with a stunning win in the women's 800m freestyle.

America's greatest ever swimmer, who was red hot favourite for the event, dominated from start to finish to claim her seventh Olympic gold medal.

Titmus had won the two previous individual races against the 24-year-old from the USA, with Ledecky having the last laugh in the fourth and final meeting between the two headline swimmers. 

Ian Thorpe said pre-race there was a 10 second difference between Ledecky and Titmus' personal best times in the event, making it a near-impossible task for the young Aussie. 

She finished with a silver in the race to take her personal haul to four medals including two gold and one bronze. 

The American led from start to finish, setting the pace and slowly building her lead each lap. 

With 200m to go Ledecky really upped the pace, increasing her stroke rate and increasing the gap between her and Titmus to 2.43 seconds with 100m to go.

Titmus, who has won two golds with incredible late efforts, was unable to reduce the gap to the American star who added another gold to her ridiculous tally. 

The 20-year-old Tasmanian, who won gold in the women's 200m and 400m freestyle over American superstar Katie Ledecky, paid tribute to her arch-rival for keeping her going. 

'I love racing Katie. It is nice to be on my own a little bit. It's been really good this week. We've been getting along really well,' Titmus said.

'We've really enjoyed racing each other. I mean, 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 two-time Olympic gold medallist has admitted she's 'absolutely buggered' as her stunning debut Olympics comes to its conclusion.

Titmus, who has become the star of the Australia's Tokyo Games, admitted she's feeling the emotional and physical strain of her first games and carrying the hopes of a nation.

'It's a big day today. I had one and a half hours in my room today. I'm absolutely buggered,' Titmus told Channel 7 on Thursday.    

'Honestly I'm exhausted. I keep thinking back to Beijing, it was the same as this morning's heats. Phelps won eight golds. 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.'