It was dubbed the race of the meet as Olympic Champion Katie Ledecky came up against the 18-year-old.
The last time the two met was during the 2018 Pan Pacific Swimming Championships with Ledecky coming away with gold in both 400m and 800m freestyle events.
Ledecky, who is just 22, has 14 World Championship gold medals in her cabinet and has never lost a major 400m race internationally.
So it was a big task for Titmus.
Titmus started in lane five after posting 4:02.42 in the heats on Sunday morning, with Ledecky taking lane four.
The Tasmanian-born Titmus started off fast and shot out to the lead until Ledecky was able to pull into the lead. It was going to do down to the wire.
However, Titmus, who has quite a strong finish in the last 100m hunted down the American superstar in the last 50 before breaking the Oceania record in the process of winning gold.
Katie Ledecky gets run down by 18-year-old Australian Ariarne Titmus in the last 50 meters, loses a major international 400m freestyle for the first time. pic.twitter.com/Vh3vMeZbTC
— Nick Zaccardi (@nzaccardi) July 21, 2019
Ledecky led by .62…
