The countdown towards what is being hyped as the fastest race in history begins on Friday as the Olympic athletics gets under way with all eyes on the prospect of a thrilling 100m shootout.For the first time since the advent of electronic timing, the field for the blue riband event of the Games will comprise the four fastest men in history -- champion Usain Bolt, Yohan Blake, Asafa Powell and Tyson Gay.Bolt rewrote the record books at the 2008 Beijing Games with a dazzling sprint double that turned logic on its head and forced a recalibration of what had previously been thought humanly possible.The good news for the London organisers is that the ebullient 25-year-old is back, professing himself ready to mount a successful defence of his 100m and 200m titles from four years ago in his bid to "become a legend"."I am the Olympic champion and I have to show the world I am the best," Bolt said on the eve of the Games. The even better news is that he will have a titanic battle on his hands, the first chinks in his armour revealed when he was beaten in both the 100 and 200m in the Jamaican Olympic trials by training partner Blake.Blake, crowned world champion in Daegu last year after Bolt sensationally false started in the final, is a serious gold medal contender. "My philosophy is that the sky's the limit," Blake said. "I've always wanted to be at the Olympics. It's everyone's dream."I've got no message for Usain Bolt. He's a good guy. I'm not focusing on Usain." Other rivals in the 100m will likely be another Jamaican, Powell and the Americans, Gay and Justin Gatlin. Gatlin won the 100m title in Athens in 2004 but was barred from defending his title in Beijing after he was banned for doping offences.Gay, who has committed to racing just the 100m in London, has made a slow start to the season, but came through the US trials alongside Gatlin, and said he was ready for the challenge after an injury-plagued period. "It's a lot of pressure, I'm not going to lie. The missing piece in my heart is an Olympic medal," said Gay, who will turn 30 four days after the 100m final on Sunday."It's really special to come here and compete for a medal. I came up short in 2008. Now I'm fully focused on these Games, and not my age or 2016, to leave with a medal." But athletics is not all about one man and his roadshow, although meeting organisers and the general public may beg to differ.Track and field will run from Friday through until August 12, the last 10 days of the Games, at the newly-built Olympic Stadium, with an estimated 2,000 athletes competing in 47 events. The men will compete in 24 events and the women in 23, with the latter's schedule lacking the gruelling 50km race walk.The US team suffered one of its worst showings in Olympics history but still finished atop the gold medal standings in Beijing thanks to its men's 4x400m relay squad winning the penultimate race of the competition.The Americans finished with seven golds, nine silvers and seven bronzes for a total of 23. Russia came second with 18 medals (six gold, five silver, seven bronze and Jamaica third with six golds, three silvers and two bronzes. Kenya bumped up their medal count to 14 (5-5-4) with gold medal-winning performances in the men's 800m and women's 1500m, and the late Samuel Wanjiru claiming the men's marathon title.The US team, always happy to have their sport centre stage every four years, will again be major contenders, with incredible strength in depth in the sprints and hurdles.On the track it remains to be seen how the Ethiopian duo of Kenenisa Bekele and Tirunesh Dibaba shape up against a formidable-looking Kenyan middle and long-distance team led by 800m world record holder David Rudisha. The two multi-medal-winning long-distance runners claimed the men and women's 5000m and 10,000m doubles within a day of each other.