Torri Huske stunned American teammate and world record-holder Gretchen Walsh to clinch the women's 100m butterfly gold medal on Sunday.
Walsh turned first but Huske chased her down to touch in 55.59secs, with Walsh second in 55.63 and China's Zhang Yufei adding bronze to her Tokyo Olympic silver.
Canadian defending champion Maggie Mac Neil finished fifth and Australia's Emma McKeon, the queen of the pool in Tokyo with seven medals, came sixth.
Huske's win was a shock, with 21-year-old Walsh heavily favoured after announcing herself to the world at the US trials last month.
Coming off a record-setting collegiate season for the University of Virginia, Walsh shattered Sarah Sjoestroem's eight-year-old world best.
But Huske has more experience of the big occasion and used it to her advantage in an arena sounding more like a football stadium.
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