Katie Ledecky raced away from the rest of the field to win the first women’s Olympic 1,500m freestyle gold.
Having competed in the women's 200m freestyle final little over an hour earlier, Ledecky won the 1,500m freestyle by about 10 metres.
The 24-year-old swimmer, who was the favourite for gold, covered the metric mile at the Tokyo Aquatics Centre in 15:37.34, which is slightly slower than the Olympic record of 15:35.35 in the heats.
Ledecky’s team-mate Erica Sullivan came home for the silver medal, giving the USA another one-two finish in the pool.
“I'm so happy to go one-two with Erica (Sullivan)," said Ledecky, who cried tears of joy.
"It is the first women's 1500m (freestyle in Olympic history) so I couldn’t have a better outcome than that. I’m so, so happy."
This race marked the Olympic debut of the women’s 1500m freestyle, which has been part of the world championship competition since 2001.
While six-time Olympic gold medallist Ledecky took the suspense out of this race by cruising into a commanding from the first 50 metres, she has not had an ideal Games.
The 1500m gold is Ledecky’s second medal of the Olympic Games in Tokyo, following her silver in the 400m freestyle. She finished fifth in the 200m freestyle.
Ledecky’s results gave the American her first taste of defeat at an Olympic Games.
“People maybe feel bad that I'm not winning everything, but I want people to be more concerned about other things in the world. People are truly suffering.
"I'm just proud to bring home a gold medal to Team USA.”
"After the 200 (metres freestyle final, where she finished fifth), I knew I had to turn the page very quickly, and in the warm-down pool I was just thinking of my family.
Each stroke I was thinking of my grandparents. They're the toughest four people I know and that's what helped me get through that."
Sullivan, who was in fifth place at the halfway stage, clawed her way back past her rivals to finish with the silver in 15:41.41.
It was part of her plan to swim a stronger second half to the race, according to Sullivan.
“Luckily I did plan to swim it that way and thank God it paid off, because there was a point there where I was worried I was too far back. I'm really glad I was able to bring it home.”
On coming home behind Ledecky, she added, “She's such a cool human ... She's a legend and she'll forever be a legend. The fact that I get to swim in the same generation as her, it's just so cool.”
Germany’s Sarah Kohler finished a further 1.60 seconds behind to claim the bronze.