Saviours of the track come in different sizes. Extra large for Usain Bolt; extra petite for Allyson Felix. The American sprinter, who has captured the imagination for her slender physique in this age of muscularity, cast off her recent patchy form to qualify for the 200 metres final at the Bird's Nest Stadium.

She will be a hot favourite to win her first Olympic gold medal in the final on Thursday, having qualified comfortably in 22.33sec, fractionally slower than Veronica Campbell-Brown, the winner of the other semi-final. Felix is a member of “Project Believe”, a group of athletes who have volunteered to be tested out of competition to try and combat the negative image of their sport. Yet a mere glance at Felix - all 5ft 6in and 8st 3lb of her - does inspire belief. Her childlike grace is exactly what makes her running so appealing and her story so persuasive.

There is a magnificent photograph of Wilma Rudolph crossing the line in the 1960 Olympics. The picture perfectly captures the long-legged elegance of the great American but it could be Felix, the high-school star writ large; the girl who didn't make the cut for the cheerleaders at high school in Colorado and who missed her prom night because of a track meeting the following day, the girl nicknamed “Chicken Legs” by her team-mates. The ordinary girl with the ordinary suburban family and the extraordinary talent. Bob Kersee, Felix's coach, says she reminds him of Rudolph.

But it's asking a lot of a 22-year-old to shoulder responsibility for so much of the past. She has enough to worry about in the future. Project Believe is a noble attempt to speak out, but the burden of proof has shifted; athletes are guilty until proved innocent in these tainted days. Felix is at the forefront of the movement, along with Tyson Gay, the 100m sprinter.

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"It means we're tested more regularly," Felix explained. "They get a base line and then go back and take more tests. It shows where our heads are at, but more than that it shows we're willing to go above and beyond the normal testing to show that we're clean."

Felix feels a duty of care to her sport because her own inspiration was Marion Jones, the disgraced Olympic champion. Felix has watched the unravelling of Jones, who is serving a six-month prison sentence for lying to Federal investigators about taking performance-enhancing drugs, with a horror bordering on betrayal. Felix broke Jones's national high-school record in the 200m five years ago, the culmination of an outstanding junior career.

"I had posters on my wall and everything that young people do with their heroes," Felix said in London recently. "So everything that came out about Marion was pretty devastating on a personal level.

"What she had accomplished meant a lot to me. I still had a little bit of hope left when she started denying it, then when everything finally came out, I finally knew the whole truth. Now when anyone runs a phenomenal time, it is questioned. It's really unfortunate."

Felix planned to run 100m and 200m in Beijing, but failed to qualify for the shorter event at the US trials, leaving the 200m, in which she won silver in Athens, as her sole individual event. She will, though, contest both the 4x100m and 4x400m relays, giving her chance to repeat her triple gold medal haul from the World Championships in Osaka. Recently, Felix has mislaid the form which captivated a sport at war with itself.

"Allyson is still an infant in this sport," says Kersee, who coached Florence Griffith-Joyner to the 200m world record and Olympic gold. "Her starts have to get better and they will, but her stride length gets better as she gets going. Once she gets into the race that fluent, gazelle, running style of hers is going to take over."

Kersee is routinely surprised by the work ethic of his brightest talent, by her ability to absorb the punishing routines that make champions. Felix points to her religious upbringing and the two-hour sermons she listened to each Sunday at the Baldwin Hills Church in Los Angeles. She had to listen; her father, Paul, was the preacher. When Felix is away at a meeting, her mother, Marlean, sends her CDs of her father's sermons, just in case she might forget her roots.

Jamaica and the United States dominate the field for the final, with Felix joined by her compatriots, Marshevet Hooker and the equally slight Muna Lee, against a trio of Jamaicans, led by Campbell-Brown, the Olympic champion. If Felix prevails, the world might start believing again.