Richards lost out to Christine Ohuruogu at the Olympics
American sprinter Sanya Richards ran the fastest women's 400m of the season at the Golden League opener in Berlin in a time of 49.57 seconds.
With the world championships just two months away, Richards threw down the challenge to Britain's 400m Olympic champion Christine Ohuruogu.
"I came here to run fast and I am looking forward to coming back and leaving with the gold," said Richards.
Britain's Simeon Williamson was second in the 100m in 10.13 seconds.
Antigua's Daniel Bailey was the winner with a time of 10.03 seconds.
"I came here to win this race, so I can't be disappointed," said Bailey.
Elsewhere, Russia's Olympic champion Yelena Isinbayeva won the pole vault.
World record holder Isinbayeva won with a modest 4.83 metres after failing to clear higher attempts.
Olympic champion Kenenisa Bekele of Ethiopia shook off any injury concerns to win the 5,000m race in 13 minutes 0.76 seconds.
German Ariane Friedrich set a new national record of 2.06m in the high jump and came close to equalling the world record of 2.09m.
The world athletics championships begin on 15 August in Berlin.