Nominal GDP is GDP before adjusting for inflation.

Real GDP is GDP after adjusting for inflation.

let's say, for next 10 years, USA unable to keep inflation under control , its nominal GDP numbers will be large while the real GDP growth rates will be small or negative. And if China can keep inflation under control its nominal GDP will not grow like USA but its real growth rates will be far higher than USA.  I expect China is able to maintain real GDP growth rates of 5-6% annually for next 10 years. 

In this scenario, also the most likely scenario, USA will remain the largest economy , nominally. But, at its core, will be rotten as a result of high inflation. In 10 years, USA will still be number one measured by nominal GDP , China will still number 2 but a much healthier economy. 

And that is what it is. China is not  in a hurry to be known as the number one economy.  In fact, for China controlling inflation is a higher priority than chasing after GDP numbers. GDP also hides many things not show in GDP numbers and are very important to place close attention...........eg blue sky green mountains and clean water or the environment. , wealth and income equality, culture, happiness, stability, education, health, housing,  

Chasing after GDP is only one among many other good objectives.