Defending Olympic marathon champion Mizuki Noguchi has pulled out of the event in Beijing with a thigh injury.
|Mizuki Noguchi of Japan celebrates her 2004 Athens success [EPA]
The 30-year-old underwent a second MRI scan after cutting short her altitude training in Switzerland, citing muscle soreness and fatigue.
The results were not encouraging and the Japanese runner has reluctantly decided to withdraw, less than a week ahead of the race.
The news is a huge blow for Japan, where marathon running is extremely popular and draws enormous crowds.
The country has won the last two women's Olympic marathons.
This year's race, due to take place on Sunday, has been split wide open, with world record holder Paula Radcliffe also not certain to even make the start line.
The British runner has had her preparations badly set back by a stress fracture in her leg that, though now healed, has left her short of distance on the road in the build up to the games.
Radcliffe said earlier this week that she intends to run "unless my leg breaks down'' as she prepares to compete at her fourth Olympic games.
"The aim has always been to start the race," she said. "Everyday I go out and plan to start the race."
"I'm getting more and more confident because every day you run more on it and it gets stronger.''
Radcliffe dropped out of the marathon at the 2004 Athens games, but she won the world championship title in the event a year later in Helsinki, Finland.
"I feel good. I feel happier even to be at this stage,'' Radcliffe said.
"I know from the stuff that I've done that I'm in good enough shape to be on
the start line.''
"I don't know but I guess I'm probably not in 2:12 shape, but I think if you put yourself on the start line, you're in with a chance,'' Radcliffe said.