Team USA's Simone Manuel, who won silver in the women's 50m freestyle at the Rio Games, missed the event's Olympic final after an 11th-place finish in the semifinals.
It's likely Manuel's last race at Tokyo 2020, her second Olympics following a four-medal performance in Rio.
Shortly after, Manuel, who battled a condition known as overtraining disorder in the leadup to Olympic Trials and had to take time away from the pool, wrote via Twitter that she is proud of her perseverance just to make it to Tokyo.
"I can confidently say that I'm a champion," Manuel wrote. "Not because of the medals I've won but because of how I've consistently fought for what I believe in, my perseverance, and my fiery passion to always be me!"
Compatriot Abbey Weitzel, who touched down in 24:19, took second in the semi and will appear in the finals.
Australian winner Emma McKeon won the race in an Olympic Record time of 24:00 flat.