Mollie O’Callaghan has pipped fellow Australian Ariarne Titmus on Tuesday morning (AEDT) to win gold in a blockbuster women’s 200m freestyle at the Paris Olympics.

O’Callaghan thwarted Titmus’ bid to become the only swimmer in Olympic history to defend titles in both the 200m and 400m freestyles.

The 20-year-old touched the wall just over half a second ahead of her pool rival, setting a new Olympic record as the nation held its breath on the last lap of the tight “race for the ages”.

“But it’s the newcomer, she’s blown her away,” cried Nine’s Olympics commentator Mat Thompson as O’Callaghan held her lead in the final metres.

“A great one-two.”

O’Callaghan clocked one minute 53.27 seconds at the Paris La Defense Arena with Titmus (1:53.81) taking the silver medal.

Mollie O’Callaghan touches the wall ahead of Ariarne Titmus. Photo: Getty

The competitors embraced in the pool as O’Callaghan soaked up her new reality as an Olympic champion — her first individual Olympic gold medal.

“Arnie, she is an absolute gun,” O’Callaghan told the Nine Network.

“She races like an absolute beast and it’s an honour to train alongside her and have such a great team around us.

“That was an amazing race … I did it for the country, I didn’t do it for myself.”

The gold medal adds to O’Callaghan’s three relay golds — two at the Tokyo Games three years ago and the 4x100m freestyle in Paris on Saturday night.

O’Callaghan was third with 50 metres remaining but powered to victory with a last lap some 0.66 seconds faster than Titmus, who made the last turn in second spot.

Her triumph delivers Australia a fifth gold medal at the Paris Games, including three at the pool.

Mollie O’Callaghan and Ariarne Titmus embrace after their duel. Photo: Getty

Meanwhile, Australia’s defending women’s 100m backstroke champion Kaylee McKeown was the second-fastest qualifier through the semi-finals.

McKeown, seeking to repeat her golden 100-200m backstroke double at the Tokyo Games three years ago, clocked 57.99 seconds – just two hundredths of a second behind her chief rival, American Regan Smith.

The Dolphins’ 18-year-old Olympic debutant Iona Anderson (58.63, ranked fourth) will also feature in Tuesday night’s medal race.

Fellow Australian Ella Ramsey finished fifth in the women’s 400m individual medley final won by Canada’s Summer McIntosh.

And compatriot Max Giuliani was seventh in the men’s 200m freestyle final won by Romania’s David Popovici.

In Monday night’s other finals, which didn’t feature Australians, Italy’s Thomas Ceccon won the men’s 100m backstroke and Tatjana Smith from South Africa claimed the women’s 100m breaststroke gold medal.

-with AAP