A mere 13 days after undergoing knee surgery, Australia's Jana Pittman came through the first round of qualifying for the Olympic women's 400 metres hurdles unscathed on Saturday.

The 21-year-old world champion thought her Olympic dream was over when she tore cartilage in her right knee warming up at the Zurich Golden League meeting on August 6.

Two days later in London she underwent arthroscopic surgery and on Saturday, after starting gingerly, Pittman won her heat in a comfortable time of 54.83 seconds.

"I was very nervous at the start," she told reporters. "Usually I'm relaxed. The question was whether the knee would hold up over the first six hurdles.

"I got through seven and then I relaxed. I was telling myself, 'Oh, yes it's going all the way.' I can't believe it. I feel like I am the luckiest woman alive.

"You don't know how much you love something until you could have it taken away."

Pittman said she would have to ice her knee for four hours after the race "to make sure it does not blow up".

"It was quite hard work for my knee, even though it was not a hard run for me, it was hard for my leg," she added.

"I had to be careful not to stutter before the hurdles because that braking force could have hurt it. That was the scariest part really."

Morocco's Nezha Bidouane, the 2000 Olympic bronze medallist and twice former world champion, failed to qualify after finishing third in her heat with only the first two sure to go through.

The 34-year-old Bidouane, who has suffered a succession of injuries over recent years, clocked 55.69 and failed to go through as one of the six next fastest.

The semi-finals of the 400 metres hurdles are on Sunday.