Paula Radcliffe has identified Mizuki Noguchi as a main rival for the 2008 Olympic marathon title in Beijing.
Noguchi took gold at Athens 2004 and looks good to challenge again
Mizuki, 29, marked her return from injury with a time of 2 hrs 21 mins and 37 secs to triumph in the Tokyo Women's International Marathon.
The Olympic champion is yet to be named in the Japanese team for Beijing but now looks assured of a place.
"I haven't raced against Noguchi for a long time, but certainly she's one of the top contenders," said Radcliffe.
"The depth in Japan is so deep her place isn't even guaranteed, even though she is one of the main contenders."
Noguchi was competing in her first marathon since setting a national record in the Berlin Marathon in October 2005.
Having recovered from a series of leg injuries, she clocked the world's second fastest time of the year.
"She ran very well," said Radcliffe, who won the New York Marathon on 4 November having herself had a long lay-off because of injury and the birth of her first child Isla.
"She's been very unlucky really, having done all the preparations and then having to pull out on the eve of her last two marathons.
"Noguchi was put under pressure in Tokyo - she's not on the team but I was very impressed.
"She can now relax a little bit, while the others are still fighting for selection."
Radcliffe has yet to return to full training following the success in New York, or confirm her competition and training schedules prior to Beijing.
London Marathon organisers are keen for her to race over the course where she set her world record in 2003.
Meanwhile, the British selectors would like to include her in their squad for the World Cross Country Championships in Edinburgh a fortnight before London.
"The only date written into the diary in Beijing, " said the 33-year-old. "Before then I haven't formulated any plans - yet.
"I'll see how it goes when I get back into training. We're at the stage really of thrashing things out and playing around.
"I want to race because I missed racing, but at the same time I want to get things right in Beijing, so I have to work out which races will be best to do that.
"The first decision will have to be whether to run a spring marathon. And then, yes or no, you look at whether the world cross country fits in to that. And which other races if that's not an option."
• The majority of Team GB will prepare for the Beijing Games at a training camp in Macau, the British Olympic Association announced on Wednesday.
Macau is situated in the same time zone and offers similar climate conditions to Beijing.
Competitors who will not train there because of specific include those from sports such as canoeing, rowing, equestrian, cycling, diving, equestrian, modern pentathlon, sailing, swimming, tennis and triathlon.