American Aries Merritt stormed to gold in the 110 metres hurdles at the London Olympics, holding off world champion Jason Richardson to cross the line in a personal best time of 12.92 seconds.

The 27-year-old world indoor champion made the most of perfect conditions on a balmy evening at the Olympic stadium to run his fifth sub-13 second time in the last two months.

Merritt made an excellent start but was being shadowed by defending champion and world record holder Dayron Robles until the Cuban pulled up after jumping the sixth hurdle with what looked like a hamstring problem.

Merritt charged on and, with Richardson gaining on him with a late surge, dipped low to cross the line first in the fastest time in the world this year.

"The gold medal means everything," Merritt told reporters. "The U.S. haven't had a gold medal since Allen Johnson in 1996. It's phenomenal. To be here in this atmosphere is really special."

Richardson won silver for the United States in 13.04 seconds and bronze went to Hansle Parchment, who crossed the line in a Jamaican record time of 13.12.

Merritt has often been overshadowed by better-known compatriots like David Oliver and Richardson but hit a rich vein of form this season.

After beating former world record holder Liu Xiang at the world indoor championships to claim his first major title, Merritt really hit his stride in the run up to the Games.

His first run of 12.93 seconds won him the U.S. trials and he backed that up with two more runs in the same time. He added the fourth (12.94) to reach the final on Wednesday.

Robles, who has barely competed this year because of a long-standing back problem caused by his flat feet, slapped out at a hurdle in frustration at being unable to coax his body through two races in two hours.

Disqualified at the world championships in Daegu last year for impeding Liu in the final, Robles has lost both his major international titles and may now consider retirement.

China's Liu, who won the title in Athens eight years ago, fell in his first-round heat after injuring his Achilles tendon.