LaShawn Merritt returned the United States to the top of the 400 metres podium at the world championships on Tuesday with a commanding performance as defending champion Kirani James faded badly to finish a surprise seventh.

Merritt took the race on from the gun and went out hard over the first 200 but, unlike James, the American had plenty left in the tank coming into the home straight to seal victory in a world-leading 43.74 seconds.

Compatriot Tony McQuay was second in 44.40 and fast-finishing Luguelin Santos of Dominican Republic edged out Belgium's Jonathan Borlee for bronze in 44.52.

Merritt was world champion in 2009 but finished behind the young Grenadian in Daegu two years ago. He is the only man to beat James this season.