Sanya Richards-Ross of the United States won gold in the women's 400 meters on Sunday.
The American posted 49.55 seconds for first place, with Britain's defending champion Christine Ohuruogu finishing strongly for silver in a season's best of 49.70.
"This win is impossible to describe," said Richards-Ross, who won 4x100 meters relay golds in Athens and Beijing.
"I worked so hard for that and I prepared for this moment over and over for the last four years."
Ohuruogu left her kick too late.
"The line came too early," she said. "It's a shame I can't defend my title, I'm sad about that."
America's DeeDee Trotter took bronze in 49.72.
