LONDON: Usain Bolt eased into the 100 metres semi-finals on Saturday but was served notice that he would have to be at his best to retain his title when Ryan Bailey and Justin Gatlin stormed to the fastest ever times in the first round at an Olympics.

World record holder Bolt, showing no signs of the hamstring injury which hampered him at the Jamaican trials, stumbled at the start of his heat but recovered to win it easing up in 10.09 seconds.

“It was good,” Bolt told reporters. I'm happy, my legs are feeling good so I'm happy.”

“My reaction was good but I made a bad step and I stumbled a little. But overall it was good.”

Bailey, the least known of the three quick Americans vying for the title, was the quickest qualifier with his blistering run of 9.88 seconds to match his personal best.

“9.88, I wasn't expecting that,” he said.

“There's no pressure on me whatever. Nobody expects me to medal so I can steal a medal from someone else.”

Gatlin, who has served a four-year doping ban since winning the title at the Athens Games, had just set the standard with his run of 9.97 in the second heat.

“I had a couple of tears before I got on the track,” he said.

“It's good to be back.”

The other American, Tyson Gay, also eased through by winning the first heat in 10.08 seconds and the second fastest man of all time said he had come through the race comfortably despite testing his recently injured groin.

“I didn't feel any pain,” Gay said.

“I did what I wanted to do and from 60 metres I really had a go.”

Jamaica's world champion Yohan Blake took the foot off the pedal 10 metres from the line but still won heat six in 10.00, summing up his feelings with the one word, “good”.

The third Jamaican contender, former world record holder Asafa Powell, was given a race by young British sprinter Adam Gemili but went through to Sunday's semi-finals in 10.04.

Gemili progressed with an impressive 10.04 and Briton Dwain Chambers, apparently forgiven for his doping past by a partisan packed house at the Olympic Stadium, won the last heat to qualify in 10.02.

Kim Collins, who had hoped to run in his fifth Olympics, failed to make the starting blocks after an apparent falling out with the St Kitts and Nevis Olympic Committee.

“My fans. I won't lie,” the 36-year-old tweeted on Saturday.

“Won't be running later tonight. For those who saw me run in Mexico. That's the last time I represent my country.

“Even men in prison get their wives to visit. 6 athletes and 9 officials. That ain't enough to make some people happy. Omg.”