According to Wikipedia:
Rat’Lar, the maniacal leader of a species of aliens called the Scrubb, demands that Earth’s greatest champion fight the greatest Scrubb fighter. If Earth refuses, the Scrubb and their huge armada of spaceships will destroy it. Superman and Muhammad Ali each come forward to volunteer. However, Ali argues that Superman is not really of Earth, and has an unfair advantage in his many superpowers. In typical Ali-style verbiage, he puts himself forward as the obvious choice.
Intrigued, Rat’Lar decides that Superman and Ali should fight one another to see who really is Earth’s champion. To make the fight fair, he decrees that the match should take place on his home planet, Bodace, which orbits a red sun (which temporarily robs Superman of his powers). The winner would simply be the best boxer. [link]