LaShawn Merritt ran a tactically perfect race to lift the United States back to the top of the 400 metres podium at the world championships on Tuesday as defending champion Kirani James faded badly to finish seventh.
Merritt took the race out from the gun and went 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.
The 2009 champion held a slight lead over James as the pair came off the final bend but while Merritt seemed to have endless reserves of energy, James's suddenly ran out and he went backwards through the field.
"It was poor execution on my end, I need to go back to the drawing board and think again," James told the BBC.
"I didn't come to the world championships expecting this. I just need to move on now."
Merritt's compatriot Tony McQuay made it a one-two for the U.S. who have now won all but four of the men's world titles in the event, in 44.40.
Fast-finishing Luguelin Santos of Dominican Republic edged out Belgium's Jonathan Borlee for bronze in 44.52.
"It isn't a shock that Merritt won the race, it is the way he won the race and the way James lost it," world record holder Michael Johnson told the BBC.
"James didn't want to run too fast too early but he had to, to keep up. Kirani had nothing left to give at the crucial moment."
Having finished second to the young Grenadian in Daegu in 2011, Merritt watched his Olympic crown go the same way last year after pulling up with a hamstring injury in his heat. He is the only man to beat James this season.
Merritt returned from a 21-month doping suspension in 2011 after a positive test for using an over-the-counter male enhancement product that contained a banned substance.