But there are signs of trouble. Except for top-tier fights such as the De La Hoya-Mayweather bout, few boxing matches ever sell out— that’s in marked contrast to recent M.M.A. events. And while M.M.A. has been zooming ahead in the important pay-per-view battle, professional wrestling has been losing ground. This month, its free distribution also took a hit, with the CW network choosing not to renew W.W.E.’s long-running Friday-night SmackDown. W.W.E. did not return requests for comment.

Desertions to the mixed martial arts ring by well-known practitioners of boxing and professional wrestling may be the most worrisome sign.

Known to professional wrestling fans as “The Next Big Thing,” Brock Lesnar made his U.F.C. debut earlier this month. As other pro wrestlers like “Stone Cold” Steve Austin and the Undertaker looked on from the sidelines, Lesnar was taken down one minute and 30 seconds into the first round, but he has vowed to fight in the cage again another day.

A more explosive move may be in the offing. Mayweather, whose 2007 pay-per-view brawl with De la Hoya brought in $134 million, has gone from talking trash about the sport—calling it “nothing but a fad” and comparing it to “a street fight”—to recently discussing entering the M.M.A. ring with  Mark Cuban’s HDNet.

Royce Feour, who spent 37 years writing about boxing for the Las Vegas Review-Journal, doubts Mayweather would ever leave his very successful and lucrative boxing career to take a chance in mixed martial arts, although a well-funded promotional bout wouldn’t be beyond the realm of reason.

That would further cement M.M.A.’s place in the world of combat sports. But even if it doesn’t happen, Feour says fans of all stripes should probably be thankful M.M.A. has lit a fire under promoters and made them pay closer attention to their audiences.

“It has been a good year for boxing because they are letting their best fighters fight each other,” says Feour. That wasn’t happening before M.M.A. burst onto the scene, “so it is definitely having a beneficial effect, whether or not anyone wants to admit it.”

Also on Portfolio:
Boxing's Great Magazine Hope
In Charity, a Heavyweight
Sideline: Patrick Ledford