The answer depends on how one wants to think of economic supremacy. If by this one means "which country has the largest GDP," then China has already surpassed the US, at least in terms of real GDP. But that's not a very useful measure, since not all economic activity is globally relevant. A country that is still at a stage of economic development where most citizens are primarily concerned with being able to afford basic necessities may nevertheless have a large GDP if it also happens to be highly populated. But if the majority of citizens are barely scrapping by, it doesn't really matter to the rest of the world how wealthy the country as a whole is. Such a country may as well be located on a different planet, unless it decided to use its large GDP to apply military pressure.
It's very likely that China will eventually surpass the US in terms of nominal GDP as well, but I think the more important question is: "Will it be an economic center on par with the US?" There too, I believe the answer will hinge on one's intereptation, since I think the US will continue to outproduce China in terms of e.g., Nobel Laureates, for at least the next 50-100 years.