The 20-year-old triumphed in a titanic final on Monday, defeating the United States' five-time Olympic champion Katie Ledecky.

Titmus' achievement is Australia's second gold medal of the Tokyo Games, following the women's 4x100m freestyle relay success.

And it delivers Ledecky, regarded as the greatest female swimmer ever, her first defeat in an Olympic final.

Titmus, in lane three, kept watch on Ledecky in lane four in what became the utimate match race between the pair.

The American held the lead for the initial 300m but Titmus was watching her all the way, literally lurking at her heels.

The young Australian ominously surged closer and was just 0.16 seconds behind Ledecky with 100m remaining.

Titmus then reeled in her rival in a perfectly-executed race plan to win by half a body-length in a time of three minutes 56.69 seconds.

Ledecky touched home in 3:57.36 and China's Li Bingjie was well back in third position in 4:01.08.

Titmus and Ledecky will also square off in a much-hyped 200m freestyle battle, though the American is favoured to win their duel over 800m.

Australian Associated Press