Maurice Greene's bid to retain the Olympic 100 metres crown got off to a solid start when he clocked 10.18 seconds to win his first-round heat on Saturday.

But Greene looks set for some hot competition, with fellow-Americans Shawn Crawford and Justin Gatlin and Jamaica's Asafa Powell among the most impressive qualifiers on a track already producing a string of fast times.

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. He was untroubled in the eighth of 10 heats held on a brutally hot day.

"It was an early morning jog, I was trying to conserve as much energy as possible," Greene said.

"It was good and it's only going to get faster."

Crawford, the fastest man in the world this season with 9.88, was the fastest qualifier with 10.02, despite running in a back-to-front baseball cap and sunglasses.

"It's the first time I've worn a hat in competition but it's just a little sun visor on the back of my neck so my engine doesn't overheat," he said

"It's the easiest 10.02 I've run in my life."

Gatlin, second in the United States trials behind Greene, said he was also cruising as he clocked a smooth 10.07.

Powell, who has beaten Greene twice in the last month and run five sub-10 times this year, was not far off a sixth as he clocked 10.06 without ever seeming to reach top gear.

Francis Obikwelu, Nigerian-born but now representing Portugal and the other man to beat Greene this year, won his heat in 10.09.

Namibia's evergreen Frank Fredericks, twice a silver medallist in the event and now six weeks short of his 37th birthday, had got things moving by winning the opening heat in his fastest time of the year -- 10.12.

World champion Kim Collins of St Kitts and Nevis won the final heat in 10.11.

Among those failing to advance were Trinidad's Ato Boldon, silver medallist in Sydney and third in Atlanta, who managed only 10.41, and Briton Darren Campbell, sixth in 2000 and the Sydney 200 metres silver medallist, who ran 10.35 after struggling this week with a hamstring strain.