Arch rivals Maurice Greene and Asafa Powell went head to head in the Olympic 100 metres on Saturday but Greene's narrow victory counted only as a psychological one as it came in a second-round heat.

Defending champion Greene won the final heat of the night in 9.93 seconds but Jamaican Powell, who has beaten the American twice in the last month, eased through behind him in 9.99.

Powell tried his best to score a mental point by slamming on the brakes at 50 metres and looking across at his illustrious rival before picking up the pace again to ease in behind him.

Greene is aiming to become the first man to finish first in successive Olympic 100 metres finals -- Carl Lewis retained his title in 1988 only after the disqualification of Ben Johnson -- but he has other threats beyond Powell.

The night was littered with sub-10 second times, led by American Shawn Crawford's 9.89.

Crawford, the fastest man in the world this season with 9.88, had been the fastest in the morning first-round heats with 10.02.

On a balmy evening he improved that to 9.89 to take the second heat ahead of Sydney bronze medallist Obadele Thompson of Barbados, who also qualified with 10.12.

Francis Obikwelu, Nigerian-born but now representing Portugal and the other man to beat Greene this year, had set down the night's marker when he won the opening heat in 9.93, despite easing down over the last 15 metres.

World champion Kim Collins of St Kitts and Nevis went through safely but looked a touch off the pace in clocking 10.05 behind Ghana's Zakari Aziz (10.02) in the fourth heat.

Collins though was unconcerned. "It makes no sense to break a record in the second round but let them have their fun," he said.

Namibia's Frank Fredericks, twice a silver medallist in the event and now six weeks short of his 37th birthday, finished fourth in the same heat in 10.17 to bow out.

The semi-finals and final are on Sunday evening.