The only "nearest" challenger of any value is Japan (but realistically not a chance as Japan would be left behind as years goes by). It could be 2 decades or more before China reaching parity with US depending on its economic planning success to turn itself into high income country, but its a very formidable task looking at the population and size of the country. Securing raw materials and energy for future growth, developed the necessary technological know how and innovation, having a well educated population, increasing urbanization rate. By then China would be a quite a different...having 4 times as large an economy comparing around her neighbours.
Projections are simply just that, speculation.
I don't think that China will follow the USA's path; China is already much more urbanized than any other country in the world, with the Greater Beijing area (Beijing, Tianjin & Hebei) having around 100 million people; the guangzhou area (Guangzhou, HK, shenzhen, foshan, dongguan) is 50 millions; greater shanghai area is around 100 million people.
But yeah, educated population, energy resource security; is all important, but the future is still uncertain.