In 2030, the top five economies on a PPP basis will be China ($30.6 trillion), the U.S. ($23.4 trillion), India ($13.7 trillion), Japan ($5.8 trillion), and Russia ($5.3 trillion). By 2050, the top five will be China ($53.9 trillion), the U.S. ($38 trillion), India ($34.7 trillion), Brazil ($8.8 trillion), and Russia ($8.1 trillion).
In 2011, the U.S. topped the charts with GDP of $15.1 trillion, followed by China with $11.3 trillion in GDP.
The changes are due largely to faster growth rates in the 7 largest emerging economies, which are expected to post total GDP 50% larger than the 7 largest developed economies by 2050.
The PwC report is available here.