Well....Japan is still the 3rd largest economy in the world. And was second for a long time. And while they fell on hard times, and still are in it, they are expected to be the 3rd largest economy up til 2020.
Japan's limitation is that they are a nation of only 125 million people vs. the US which has about 310 million people. And China, of course, is about 1.35 billion people. Those factors need to be kept in mind.
Well, they won’t have 1.3 billion people for long, not with a fertility rate of 1.6. Trying to project populations decades into the future is pretty dubious, that I know, but some of the more credible estimates put China as having crashed to under 1 billion by 2100, and the American population at more then 500, 600 million.