We rule it insofar as anyone does.
We have a per capita GDP 7 times that of China, and first among large countries (with a population of 50 million or more).
China poses exactly zero physical military threat to us at the moment (but their cyber attacks and corporate hacking have been a drag on our economy).
We still police the world, and that seems a pretty good proxy for ruling.
In most instances we just have to send one carrier group somewhere to stabilize volatile international situations.
Certainly we aren't as dominant as we once were, but we have no peers in either economics (China's economic troubles with its real estate boom) or militarily.