Felix targets 200m gold

"I ran very poorly in Stockholm and London because I was just exhausted," the 2004 Olympic silver medallist said on her online blog.

"Now, I'm feeling great and where I need to be," said Felix, who has been working out at the American training centre in Dalian, northeastern China. "I'm emphasizing my start and working on my curve running, along with keeping up my endurance."

The race between Olympic champion Campbell-Brown and world champion Felix could be one of the highlights of the women's track events at the Games.

Campbell-Brown beat Felix by 0.13 of a second in Athens, but the American sprinted to a half-second triumph at the 2007 worlds, clocking 21.81 seconds.

"I've learned so much from my first Olympics in 2004," said the 22-year-old Felix. "I'm definitely more relaxed this Olympics. I know what to expect and I'm very confident in my training."

Felix won the U.S. Olympic trials in early July, then dashed off to Europe.

"My legs were simply dead," she said. "My coach Bobby Kersee and I debated running the 200m in London because I was feeling fatigued so when I came off the curve and realised I couldn't shift to another gear I just cruised in as to prevent injury."

She flew home the next day and has not raced since, working instead on sharpening her skills for the showdown with Campbell-Brown.