Caster Semenya, a South African middle-distance runner, is the latest Cinderella story to come out of the London Olympics.

Semenya, 21, made her Olympic debut on Wednesday morning, finishing an impressive second in her preliminary heat in the 800 meters. The South-African's London 2012 appearance comes three years after she was forced to undergo gender tests that at one point seemed to cast doubt over her future in track and field.

"It was a very important race," Semenya told reporters. "It was a tactical race. I wanted the race to be a fast one. To be a good contender, you have to run under two minutes."

In 2009, she won both the 800-meter and 1,500-meter races with the times of 1:56.72 and 4:08.01 respectively. The 800 time was the record at that date.

Later that year Semenya won gold in the 800 meters at the World Championships with a time of 1:55.45 in the final, again setting the fastest time of the year.

Following her victory at the world championships, questions were raised about her gender. Semenya was sidelined for 11 months while track and field's governing body decided whether or not to allow her to compete after posting the incredible winning time.

She was tested and eventually cleared to compete in 2010, but she struggled with a back problem for a while before returning to the world championships at Daegu and winning a silver medal.

Semenya was eventually selected to carry the flag for her country at the opening ceremonies at the London Games.

After posting an Olympic qualifying time at the national trials earlier this year, Semenya said "I have to win a gold."

"My dream is to win the Olympics," she said, "and that's my plan."

The South African runner completed her heat in 2:00.71, finishing just behind American Alysia Johnson Montano, who came in at 2:00.47 -- the fastest qualifying time in the preliminaries. Defending Olympic champion Pamela Jelimo, a Kenyan, won her heat in 2:00.54 to advance with the second-fastest time.

"She's just like any other competitor for us," said Canada's Jessica Smith, who also advanced to the next round Wednesday. "The faster she runs, the faster we have to compete with her."

With her mind on the completion, Smith opted to leave the gender issue to the experts.

"I have confidence in what the IAAF has done to ensure that she's competing at the right level," Smith said. "I trust in what they're doing and hopefully she's just like any of us."