It could be the first of many. As in both gold medals and world records.
Stanford incoming freshman Katie Ledecky, a teenage sensation since bursting onto the scene four years ago in London, set a world record in winning the 400 free Sunday at the 2016 Rio Olympics.
"I got lackadaisical with my last stroke," she said. "I'll be fine."
And she was, giving the U.S. it's first swimming gold and its second overall.
Ledecky, the only woman to ever break the 3:59 barrier in the event, went 3:56:46, about five seconds quicker than second place.
"It feels really good. I'm pumped. I was so close to breaking it (this morning)," Ledecky said. "The swim was almost identical to this morning, but with a bit more pop on the last lap."
She's become nearly predicable in stepping it up on the big stage, though she set a world record in the 800 free at an invitational in Woodlands, Texas last summer.
The crowd at the Aquatic Center knew they were watching something special and cheered the 19-year-old on with each passing millisecond.
Great Britain's Jazz Carlin earned the silver in a time of 4:01:23. American Leah Smith won the bronze at 4:01:92.