Tonique Williams-Darling of Bahama added the world 400m title to the Olympic crown she won last year in one of the most thrilling races of the world Championships so far on a rain soaked track at Helsinki's Olympic stadium.
The Bahamian came from behind over the last 50 meters to overcome this year's world leader and pre-event favourite Sanya Richards in 49.55 seconds, her personal best of the year.
Richards of the United States took the silver in 49.74 while defending champion Ana Guevara of Mexico claimed the bronze with her best run of the year, 49.81.
Williams-Darling's time was the fastest winning run in this event since Marie-Jose Perec in 1995. She now joins Perec and Cathey Freeman in holding both titles simultaneously.
The race was expected to be a battle between the three eventual medallists, who have taken turns dominating this event over the last three years. Indeed, with the world champion, the Olympic champion and the top six women in the world this year, the final truly lived up to its billing, as the athletes refused to be washed by the increasingly heavy rain.
Williams-Darling, in lane six, went out hard, as did Richards in two while Guevara started slowly. Williams-Darling was marginally ahead at 200m, but Russia's Svetlana Pospelova, in lane five then began to move up.
Richards ran the quickest bend and entered the home straight ahead as behind her Guevara began to fight back, desperate to defend her title. She caught Pospelova, but Williams-Darling had saved some strength and came again on the outside as the American faded for the first time this year.
The Bahamian crossed the line with a look of wonder and disbelief on her face before falling to her knees and kissing the sodden track.