Tonique Williams-Darling of the Bahamas just edged rival Ana Guevara of Mexico to take gold in the women's 400m.
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Williams ran a brilliant even-paced race, taking the lead going into the last bend, and Guevara was unable to respond down the final straight.
Williams, the fastest 400m runner in the world this year, won in a time of 49.41 seconds.
Guevara clocked 49.56 while Russia's Natalaya Antyukh secured the bronze with a time of 49.89.
"I just ran the race I wanted to and it all worked out well," said the champion.
"In the home straight, I was side by side with Ana. She wanted that gold medal, but so did I."
Guevara said: "Tonique was very, very strong. That is sports.
"Some days you have great races when everything just goes right."