Australian phenom Ariarne Titmus paced herself in the opening half of the women's 200m freestyle final, staying half a body's length behind the leaders.
But in typical Titmus fashion, she surged past her rivals in the final two laps.
It was a tight finish, but Titmus managed to touch the wall ahead of Hong Kong's Siobhan Bernadette Haughey and win another gold, also setting a new Olympic record with a time of 1:53.50.
"S**t, I'm bloody exhausted. I knew Siobhan wanted to win that race and knew it would be tough to beat her," Titmus said after the win.
"It wasn't the time I wanted to swim but it's the Olympics and there is a lot going on.
"I feel good, it is crazy to think I'm only halfway through my program here I still have the relay and 800m to go.
"But I've got the rest of the afternoon off so I'm going to get some rest and be back ready for tomorrow."
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Titmus broke down in tears on the podium after receiving her second Olympic gold medal of the week.
Titmus embraced coach Dean Boxall following the medal presentation in Tokyo's Aquatic Centre, struggling to control her emotions.
"It is like a pressure cooker where it's all been building, building and you release that valve a little bit and the emotions well up," Channel 7 commentator Basil Zempilas said.
"You can't help but feel the emotion alongside Ariarne Titmus, that it's been building, building, building and she's just realised the pressure and the weight of the world has just lifted off her shoulders a little bit in the way she's realised her dream."
The 20-year-old today became the first Australian woman to win the 200m and 400m freestyle at the same Olympics Games since Shane Gould achieved the rare feat in 1972.
Australian swimming legend Ian Thorpe told Channel 7: "Just goes to show Ariarne Titmus and her composure. We have a saying that is 'trust the process.' It is when you're training and really trust the training and the program that your coach has set for you and the race plan that you have and sticking to the plan.
"So trust the process, execute the process, and Arnie did that so well. To be so composed when someone is chasing right next to you."