Many people think the 20th century as the "American Century." Actually we were a peripheral player until 1945, locked in a power struggle for the next 45 years, and was only the world's sole superpower for 9 years.
The US will remain very powerful until global institutions begin to take precedence some time in the more distant future. The US economy is almost 3 times larger than China's and 25% of the worlds eco activity happens in the US. China will grow but has a host of serious problems that will keep it from becoming anything but a large economy based on a huge population. The US population will continue to grow and have much better per-capita stats than emerging powers. The US Navy controls the seas and the military is an order of magnitude more powerful than anyone else. We have our problems and other countries will continue to grow, but don't believe the hype. Our society is dynamic, creative, and strong. Undermining the type of power the US has can happen, but it takes generations... or it'll take a cataclysmic event that takes the rest of the world down too.