Katie Ledecky smashed yet another record and made FINA World Championships history, while Sun Yang also retained his 400 metre freestyle title and Sarah Sjostrom claimed a world record in Budapest on Sunday.

Ledecky was in a class of her own yet again on the first day of swimming competition at a packed Duna Arena, bettering the World Championship mark she set in the heats with a stunning swim of three minutes 58.34 seconds.

The 20-year-old sensation is the only woman to be crowned 400m freestyle world champion three times and went on to claim an 11th world title in the 4x100m free relay.

Leah Smith made it a United States one-two in another 400m freestyle final dominated by Ledecky, with Li Bingjie taking silver for China.

Sun got one over his rival Mack Horton, becoming the second male swimmer in World Championships history to win the 400m free title.

Australian Horton caused a stir when he took aim at Sun by saying he has "no time or respect for drug cheats" before beating him in the 400m Olympic final last year.

It was Sun who was back on top of the podium in Hungary, though, as the Chinese star won in 3:41.38. Horton had to settle for silver, with Italian Gabriele Detti filling the podium.

Sjostrom set a new world record in the first leg of the women's 100m free by touching the wall in 51.71, but it was bitter-sweet for the Swede as the USA took gold ahead of Australia and the Netherlands.

It was a fruitful day for the USA as the men also took the 4x100m title, while Olympic champion Adam Peaty set a new championships record with a swim of 57.75secs to ease into the 100m breaststroke final.