The Olympic bronze medallist Sanya Richards has run the fastest women's 400 metres of the year (49.00 seconds) to win an elusive world championship gold at Berlin in 49 seconds.
The 24-year-old American finished strongly to defeat Olympic runner-up Shericka Williams of Jamaica by 32/100ths of a second.
Russia's Antonina Krivoshapka took third in 49.71.
