Race of century: Ariarne Titmus pipped by rival in 200-metre freestyle final

The 23-year-old recorded a time of 1.53.81 minutes behind O'Callaghan's new Olympic record of 1.53.27 at Paris La Defense Arena.

Hong Kong's Siobhan Haughey, who won bronze, led for the opening three laps before O'Callaghan made her move on the final turn.

The Aussie stayed under water longer than her rivals and surged into the lead from third.

O'Callaghan invited Titmus onto the gold medal dais for the national anthem in one of the most touching moments of the Olympics.

And it was lovely to see Titmus reminding O'Callaghan to hold her gold medal up for photos.

It was Australia's first one-two since Ian Thorpe and Grant Hackett at the 2004 Athens Olympics and O'Callaghan, 20, became the youngest Aussie to have won four Olympic gold medals.

"It's such an honour to be among everyone, especially Arni and compete against her," O'Callaghan told Nine.

"She is an absolute gun and she races like an absolute beast and it's an honour to train alongside her and have such a great team around us."

They are both trained by Dean Boxall and O'Callaghan, 20, was the 200m world record holder before Titmus claimed it by beating her at the Australian trials in June.

Titmus said Boxall should be so proud of his efforts.

"He's a brilliant coach, I believe the best in the world," she said.

"And to manage this and have the two fastest girls in the world is a credit to him.

"And it's not just us, on opening night, he contributed to every medal that we won as a coach.

"I just feel very blessed to have him in our corner. And if you want anyone backing you, it's Dean, and so we'll just be forever grateful for him."

It was an intriguing duel given O'Callaghan is a sprinter while Titmus excels in middle-distance racing.

They went one-two in their semi-final with Titmus qualifying fastest overall in 1.54.64 and O'Callaghan second quickest in 1.54.70.

The emerging swimmer spoke of how she had mentally prepared after losing her world record to Titmus

"You've just got to accept the challenge at the end of the day. You can either run or you can fight, and I chose to fight," she said.

"I'm always striving for more and I always put a lot of pressure on myself. My expectations are very high ... and to be honest, I did it for the country. I didn't do it for myself.

"I'm racing for all these people and I just had to put it behind me and there's less pressure now (for the rest of the Games), I get to swim freely."

It was Titmus' second overall medal of the Games after she won gold in the 400m on night one in a time of 3.57.49.

The Riverside and Launceston Aquatic Club swimmer, who is now Queensland-based, went into the race with the 200m world record in 1.52.23 minutes which she achieved at the Aussie trials.

She also held the Olympic record of 1.53.50 from her Tokyo gold.

"The 200 is a tricky one where I'm coming off the 400 and a lot of the girls are coming up from the 100 so they've got a lot more speed than me," Titmus said in the lead-up.

"So, I've just got to go out and hold on. It's tough but I've got to play to my strengths and it's been working lately."

Earlier, Hobart Aquatic Club product Max Giuliani, 21, came seventh in the men's 200m freestyle final in 1.45.57.

Meanwhile, Queenslander Kaylee McKeown has cruised through the opening race of her audacious bid to become a triple gold medallist in individual events.

But fellow Australian Sam Short has surprisingly failed to reach the men's 800m freestyle final - he was expected to be among the medal contenders.

McKeown comfortably advanced to the finals of the women's 100m backstroke with a strong heat swim and then backed it up in her semi-final.

The reigning Olympic champion clocked 58.48 seconds to win her heat.

McKeown is the third-quickest into Monday night semi-finals while compatriot Iona Anderson (59.37, ranked seventh) also progressed.

American duo Katherine Berkoff (57.99) and world record holder Regan Smith (58.45) topped times in the heats.

McKeown is seeking to defend her 100-200m backstroke titles from the Tokyo Games three years ago.

The 23-year-old has also added the 200m individual medley to her Paris program as she targets becoming the first Australian to collect three gold medals in individual events at a single Games.

Star paddler and flag bearer Jess Fox is also chasing the same triple-treat in Paris but, if successful, would achieve the feat after McKeown, who will also feature in medley relays at the pool.

In the men's 800m free, Short's struggles continued when ranked ninth in the heats in a sluggish time of 7.46.83 minutes.