Australia's Ariarne Titmus triumphed over five-time Olympic gold medallist Katie Ledecky in the women's 400m freestyle on Monday.

But Titmus managed to close the gap 50 metres before the finish, powering home for gold in the Tokyo Aquatics Centre.

"I can't believe it, I'm trying to contain my emotions," she said afterwards.

It gives the Australian the advantage in the first of three potential head-to-head showdowns between the two swimming powerhouses in Tokyo - they will face each other again in the 800m and 200m freestyle events.

However, a humble Titmus gave her rival much credit after her win.

American talisman Caeleb Dressel led his team to gold in the men's 4x100m freestyle relay final, cruising to victory with more than a second between them and the Italians in second.

It is the first of a potential six-medal haul for Dressel in Tokyo, who already has two Olympic golds to his name, won at Rio 2016.

He is competing in three individual events - the 50m and 100m freestyle, plus the 100m fly - and will feature in a further two, possibly three, relays

He praised his team-mates Blake Pieroni, Bowen Becker and Zach Apple, and immediately gave his gold medal to Brooks Curry, who swam in Dressel's place while he was rested in the heats