Gretchen Walsh bettered her own 100m butterfly world record for the third time in a year as she clocked 54.33 seconds in Fort Lauderdale, Florida, the same event where she became the first woman to breach the 55-second mark.
The American world champion now owns the 13 fastest times in the event's history.
"Must be magic or something out here... let’s make this an annual thing, shall we?," the 23-year-old Walsh posted on Instagram.
Walsh, who narrowly missed out on an Olympic gold in 100m butterfly to compatriot Torri Huske at the Paris Games, clocked 54.60 seconds last year to become the first woman to go under the 55-second mark.
Walsh is over a second quicker than the next-fastest woman, Sweden's Sarah Sjostrom, who clocked 55.48 seconds at the 2016 Olympics.