In what aspects?
Technologically, China is increasingly ahead of the US (without its strong allies like Japan, Germany, Taiwan and South Korea, the US seems to be nothing). It may not even be able to manufacture even an aircraft engine without imported precision machineries and equipment, or independently manufacture a boiler for nuclear plant without Japan, while China can. (I do not forget that the US still leads in some important areas, don't worry)
Economically, although the US is bigger than China (at least nominally), few can feel its mighty. Who is the main investor in Asia, Europe, South America and Africa?. Certainly not the US. Most of the world now depend more on Chinese market rather than the US one.
The US seems to lead in military (based on numbers of equipment) for now, but the gap is closing fast.
Politically, the US system sucks.
And the most important of all, education quality in the US keeps falling (relatively compared with China and similar countries, like Korea), while China is improving. I cannot see its improvement anytime soon, if ever.