Jeremy Wariner congratulates LaShawn Merritt (R) after the men's 400m final at the 2009 IAAF Athletics World Championships. (Olivier Morin/AFP/Getty Images)
The 23-year-old Merritt surged ahead in the home straight with fellow American Wariner, who was seeking his third successive world title, unable to keep up with his powerful opponent who ended with the best time of the year.
Rennie Quow from Trinidad & Tobago was third.
Merritt emerged from Wariner's shadow in Beijing when he won the Olympic title by the widest margin since 1896. Wariner had pledged to come back from that defeat and with a good start looked strong going into the bend.
Hitting the home stretch, though, Merritt accelerated with Wariner visibly struggling and finished some six metres ahead of his rival.
With no love lost between them, the pair, who helped the United States win a 4x400 relay gold in Beijing, will have to join up again for the relays on Sunday.
