London, England

Defending Olympic champion and 100 meter world record holder Usain Bolt allayed all fears about a hamstring injury, virtually sleepwalking to the finish-line in first place in 10.09 seconds in an early heat in the 100 meters.

Bolt's performance seemed to be to send a message to his competitors -- especially Yohan Blake -- that he would be the one to beat in one of Olympic track and field's spotlight events.

Bolt's time as not the best of the day as American Ryan Bailey clocked a personal best 9.88 seconds, the fastest ever first-round qualifying time in the event.

The 23-year-old Bailey said that he ran such an inspired race because there was no pressure on him and that he "just wanted to get out there and run".

Michael Phelps and Ryan Lochte's rivalry in the pool might just be over, but another even more interesting rivalry has been shaping up on the athletics track for the last couple of months - one that has seen Bolt's teammate Yohan Blake making a serious bid for the Olympic sprint title.

Yohan Blake clocked 10 seconds dead in his heat, while another famed Jamaican, Asafa Powell, won his race in 10.04, keeping the famed Jamaican trio in contention for medals.

The other famed American to breeze in the semifinals was 2004 Olympic champ Justin Gatlin, who sprinted to victory in 9.97sec. He later told the media, that there was plenty of inspiration for him to run really fast as the track was "super-fast".

Tyson Gay, the 2007 Osaka world double-sprint champ, too had a decent run, finishing in 10.08sec. The semis and finals of the showpiece event are slotted for Sunday evening at Olympic Stadium.

However, the weathermen have warned that Bolt and Blake might have run their 100m final in rain, with showers predicted for late Sunday.

Meanwhile, South Africa's Oscar Pistorius - famously called the blade runner because of his specially designed prosthetic legs - created history by becoming the first double amputee to compete at the Games' athletics event and also qualify for the 400m semifinals with a season's best time of 45.44 seconds.

But there was a huge setback for American quarter-miler, LaShawn Merritt, the favorite to retain the 400 title who had to pull out due to a hamstring injury.

The 26-year-old Merritt, who was trying to bring his career back on track after a 21-month doping ban, stopped around the 250 meter mark in his heat and then covered the remaining distance walking.

"It's probably a hamstring strain. I felt it twitch during my warm-up," Merritt was quoted as saying. Merritt's pullout was a double blow for the United States as he is also likely to miss the 4x400m relay.

The women's triathlon gold went to Switzerland's Nicola Spirig with a time of 1:59:48. Lisa Norden of Sweden and Australia's Erin Densham claimed silver and bronze, clocking 1:59:48 and 1:59:50, respectively.