Nobody lasts forever.
Even the greatest boxers lose a half-step of quickness, a split-second of reaction time or a bit of power from their punch. It's impossible to escape.
That may have been the cause of Manny Pacquiao's recent knockout at the hands of Juan Manuel Marquez.
Would Pacquiao have walked into a big knockout punch two or three years ago? It seems like a dubious proposition.
Pacquiao did not have an unblemished record when he stepped into the ring with Marquez for a fourth time. He had tasted defeat and been stung by punishing punches many times. But it was still a shock to see him go down so hard against an opponent whom he had a 2-0-1 record against prior to their fourth fight.
That defeat probably means that the long-discussed fight between Pacquiao and Floyd Mayweather (43-0) is a long way off.
There was some talk that Mayweather vs. Pacquiao would become reality in 2013 if Pacquiao had defeated Marquez, but that's unlikely now.
Mayweather will fight again in 2013, and a fight with Robert Guerrero could be the next one on the horizon.
Guerrero (31-1) may not be Mayweather's equal, but he has been impressive. Guerrero's last fight was a unanimous decision over Andre Berto in November (source: Los Angeles Times).
Guerrero knocked Berto down in each of the first two rounds and then took it to his opponent for the majority of the fight. Berto was bigger than Guerrero, but Guerrero was not intimidated.
That's just the kind of fire Guerrero or any other opponent would have to bring into the ring against Mayweather in order to have a chance.
But unless Mayweather has undergone a serious decline since beating Miguel Cotto in a unanimous decision last May, does Guerrero or anyone else have a serious chance against him?
Mayweather is not only undefeated, he has rarely been tested. The closest fight of his pro career was a split-decision victory over Oscar De La Hoya, but most of his fights have been one-sided.
Mayweather is the best pound-for-pound fighter in the world, and it may be a while before the 35-year-old fighter gives up that title.
Here are the reasons he will retain that honor:
No. 1, he has the skill in the ring.
No. 2, he has the athleticism to beat his opponents to the punch.
No. 3, he has the ego to stand up to any opponent and refuse to accept defeat.
Mayweather's skill is seen in the crispness of his punches and ability to show off incredible defensive skills. Mayweather's jab is sharp and quick, and he can follow with a blistering right hand at any time.
If his opponent can get in a punch or two to challenge him, Mayweather knows how to block, parry and avoid future shots.
His quickness allows him to make up for mistakes. His head movement allows him to avoid punches that appear to be on target. Punches that would take out lesser fighters may not even graze Mayweather.
Finally, Mayweather has the kind of unbridled arrogance that is rarely seen in any sport. He can't conceive of an opponent who could get the best of him over 12 rounds.
Has he slipped in any of those areas? Perhaps he might have a tougher time knocking out a top-of-the-line opponent now than he would have four or five years ago.
But the idea that another fighter is ready to challenge him and beat him at this point is still a long shot.
Mayweather will eventually find out what happens when he's in the ring with a stronger and quicker opponent.
But he's not ready to let that happen yet. He's still got too much left in the tank.