Mexico's Ana Guevara ran the eighth fastest time in history to claim the women's 400m title in Paris.
The 26-year-old controlled the race from the start and kicked in down the home straight to win in 48.90 seconds.
Jamaica's Lorraine Fenton set her season's best with a time of 49.43 to grab silver for the second World Championships running.
Defending champion Amy Mbacke Thiam of Senegal took bronze.
Guevara's classy run was the fastest time over the distance for seven years and proves why she has remained unbeaten for two seasons.
"I had to push through the last 100m but I'm very happy," said Guevara.
"It was my dream to run under 49 seconds, so I'm very emotional."
Britain's Commonwealth silver medallist Lee McConnell did well to reach the final and finished in seventh spot in a time of 51.07.
1. Ana Guevara, Mexico, 48.89
2. Lorraine Fenton, Jamaica, 49.43
3. Amy Mbacke Thiam, Senegal, 49.95
4. Natalya Nazarova, Russia, 49.98
5. Tonigue Williams, Bahamas, 50.38
6. Olesya Zykina, Russia, 50.59
7. Lee McConnell, Britain, 51.07
8. Svetlana Pospelova, Russia, 51.30