Branko Milanovic shows (using Maddison data) that:
According to Maddison's data, #US was the largest economy from 1870 to 2010. #China was the largest before and after. pic.twitter.com/hu3g2p7Wuy
— Branko Milanovic (@BrankoMilan) July 7, 2013
Note that in terms of per capita GDP China is still a relatively poor country, given differences in population size. However, from a macro point of view, Chinese demand now plays an important role in the world economy.