(CNN) -- Defending Olympic women's marathon champion Mizuki Noguchi of Japan has been ruled out of the Beijing Games because of injury.
A thigh injury means Noguchi will be unable to defend the Olympic title she won in Athens four years ago.
Noguchi, who claimed the gold at Athens four years ago, had been a doubt for the August 17 race after tearing her left thigh in training.
The 30-year-old, who was bidding for unprecedented back-to-back marathon wins, returned to the western Japanese city of Kyoto for tests on Monday, cutting short a month of training at the high-altitude Swiss resort of St. Moritz.
Noguchi was originally scheduled to arrive in Beijing on Wednesday to try for Japan's third straight win in the women's marathon after Naoko Takahashi triumphed at the 2000 Sydney Games.
World record holder Paula Radcliffe of Britain, Kenya's two-time world champion Catherine Ndereba and China's Zhou Chunxiu were seen as Noguchi's main rivals.
Noguchi has been plagued by injuries since her Olympic triumph. Her lone race in three years came last November when she won in Tokyo.
Two months ago, she spoke confidently of her prospects and talked about wearing ultra-light shoes partly made of rice husks.
"The shoes are easy to run in," Noguchi said at the time. "The Beijing course is hard on the feet and legs. It's flat and the surface of the roads is very hard."
She also spoke of trying to become the first repeat winner in the women's marathon, which was added to the Olympic program in 1984. "I don't really feel pressure," she said. "To the contrary, it makes me happy that I'm getting this much attention."
In April, Noguchi had carried the Olympic torch through Nagano, site of the 1998 Winter Games, protected by riot police from anti-China protests. "As I ran, I wished for the success of the Beijing Olympics and peace," Noguchi said.
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