Mollie O'Callaghan was a nervous wreck before thwarting her club-mate and compatriot Ariarne Titmus' quest for Olympic history.
O'Callaghan captured Australia's fifth gold medal of the Paris Games with victory in the women's 200m freestyle.
The triumph came at the expense of silver medallist Titmus, who was seeking to become the first swimmer to successfully defend Olympic titles in both the 200m and 400m freestyles.
"I know what it's like to be Olympic champion and I'm honestly so happy for her," Titmus said.
Both swimmers train under coach Dean Boxall in Queensland.
And both were in tears after Monday night's final at the La Defense Arena, where O'Callaghan overcame Titmus - and extreme anxiety.
"I was a nervous wreck ... my brain just overthinks," she said.
O'Callaghan was fifth at the midpoint, third on the last turn, and then produced a powerhouse last lap to finish in one minute 53.27 seconds.
Titmus, who won the 400m freestyle on Saturday night, was fourth after the first lap, then moved to second on lap and held that spot for the duration, touching in 1:53.81.
O'Callaghan lauded Titmus as an "absolute beast".
"She deserves so much," O'Callaghan said.
"It's incredible. This is her second medal and it's day three. Like, come on, that's amazing."
The victory is O'Callaghan's first individual Olympic gold medal, adding to her three relay golds - two at the Tokyo Games three years ago and the 4x100m freestyle in Paris on Saturday night.
"I can't explain it, there's no way to explain it," O'Callaghan said.
"I'm swimming for the country ... this medal is for my support team, my coach, my family. It's not just for me.
"There was a whole village behind me getting me to this point, so I'm very thankful to have them."
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.
