Faith Kipyegon set a new 1500m world record in the Florence Diamond League meeting on Friday, with Britain's Laura Muir a distant second.
Muir was 10 metres behind Kenya's reigning world and Olympic champion, 29, heading into the final lap.
And Kipyegon streaked clear to win in 3 minutes 49.11 seconds, breaking the 3:50 barrier for the first time, while Muir ran a season's best of 3:57.09.
Ethiopia's Genzebe Dibaba set the previous record of 3:50.07 in 2015.
Ireland's Ciara Mageean also ran a season's best time as she finished fourth in 4:00.95.
Kipyegon has won both world and Olympic gold on two occasions but this is the first time she has broken the world record.
She now holds the first and third fastest times in history, running her previous best of 3:50.37 last August.
How Kipyegon achieved the mother of all feats in Tokyo
Former 200m world champion Dina Asher-Smith had to pull out of the women's 100m moments before the race after suffering from cramp in the blocks.
"Other than that all good, but I didn't want to risk anything today," she said on social media.
"Frustrated as I was so excited to race, but had to make a sensible call for the bigger picture."
Fellow Briton Imani-Lara Lansiquot finished third in 11.16secs, behind Marie-Josee Ta Lou of the Ivory Coast (10.97) and Germany's Gina Lueckenkemper (11.09).
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