Australian Jana Pittman will go in the women's 400m hurdles heats after battling back from a knee injury.
Pittman beat American Sandra Glover to win her world title in 2003
The world champion's hopes of competing in Athens seemed to be over when she tore her right-knee cartliage but doctors have given her the all-clear.
"I got the confirmation on Wednesday that I was running," said Pittman.
"It's been a very long journey - one that was quite heart-breaking. To be here now and to be allowed to race is a dream come true."
The 21-year-old picked up the injury at the Zurich Golden League meeting on 6 August and underwent an operation in London on the following day.
Pittman has been recovering at the northern Italian town of Varese before arriving in the Olympic village.
Her coach Phil King revealed Pittman had only been over three hurdles in training since recovering from her injury.
But he remained confident her lack of sharpness will not affect her chances.
"She starts equal with everybody else on Saturday morning," said King.
"She is ready to race and has to take the attitude that she
will have a crack at this.
"As far as I'm concerned she's 100 percent."
Pittman is regarded as Australia's best chance of a track medal in Athens.
The world champion recorded the fifth fastest time - 53.43 seconds - in the world this season.