That time when Floyd Mayweather got rocked

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When all-time great Floyd Mayweather takes on pro debuting novice Conor McGregor, the UFC star immensely familiar with fighting but not boxing, most agree on one aspect of the fight: For the "Notorious" one to win, he is going to have to score a knockout.

Mayweather (49-0, 26 KOs) is a master boxer and perhaps the greatest defensive fighter of all time, so should McGregor land that big punch and win, many will consider it the biggest upset in sports history.

Throughout his 21-year professional career, Mayweather has rarely been hit cleanly and has only been knocked down once officially, so there are many who believe the result of the 12-round junior middleweight bout on Saturday (9 Showtime PPV, 9 p.m. ET) is a foregone conclusion: an easy Mayweather victory.

But there is the great "what if?" What if McGregor does land that big shot? It's boxing. It's two men throwing punches at each other's head. Anything can happen.

That's the way those involved in the event have been hyping it -- that McGregor, with a powerful left hand, is capable of knocking Mayweather out.

"Conor can punch. Conor hits hard. Conor knocks people out," UFC president Dana White said. "Listen, people think that Conor is going to be completely outclassed. Some people are saying that he will never hit Floyd once.

"I will tell you this. I just watched him work out [recently]. He looks phenomenal. He is in great shape. He hits hard, and now we are fighting in eight-ounce gloves, and this is going to be a fight. Anything can happen in a fight."

If there is anything that McGregor and White can hang their hats on, it is that Mayweather has been wobbled, even if only once in a blue moon.

Jose Luis Castillo landed consistently on Mayweather in their first lightweight title fight in 2001. DeMarcus "Chop Chop" Corley, a southpaw like McGregor, tagged him with some heavy shots in their 2004 junior welterweight bout. Zab Judah, also a southpaw, landed often in the first few rounds of their 2006 welterweight title fight. Miguel Cotto (2012) and Marcos Maidana (two fights in 2014) also found the mark here and there.

But the one man who landed the best punches on Mayweather in his entire career and legitimately hurt him was former three-division world champion and former pound-for-pound king Shane Mosley, in the second round of their 2010 welterweight bout.

Mayweather wound up winning handily, 119-109, 119-109 and 118-110 on the scorecards, but there were moments of true crisis when Mosley landed two huge right hands that had Mayweather in serious trouble. It is a piece of video that McGregor and his team have no doubt pored over hoping to replicate it.

Mayweather doesn't deny he was hurt, but he's proud that he persevered.

"I don't believe it was my best punch. I believe the first right hand, I threw it hard but not as a knockout punch. It just caught him really, really flush. The second time I threw it a little harder, expecting to hurt him when it landed, and I did. He took the shot very, very well with no problems, and comparisons between me and McGregor is not even in the same stratosphere." Shane Mosley

"One thing about me, I've been hit by some of the best. All I did was bite down and say, 'OK, it's OK,'" Mayweather said. "I got hit by Shane Mosley. Went right back to the corner [at the end of the round]. They said, 'What's going on?' I said, 'Everything's OK.' I said, 'I'll be a different fighter when I go back out there.' I lock in, I key in. I said, 'Here we go. Stick to the game plan now. Let's do what we have to do.'"

But those final two minutes of that second round were rough and give McGregor at least tangible proof that Mayweather can be hurt.

With about two minutes to go in the round, Mosley landed a clean right hand to the head when Mayweather inexplicably had his hands down. The shot rocked Mayweather, who immediately grabbed onto Mosley in an effort to stay on his feet and to smother more incoming shots.

"My thought when I landed the first right hand was I caught him with a good shot, and I was kind of expecting him to lean away to take the sting away, but he caught it pretty flush, and I rocked him. And from that moment I felt I can knock him out, and he can't take my power," Mosley told ESPN.

About 40 seconds later, Mosley unleashed an even bigger right hand. This one caught Mayweather on the side of the head, and Mayweather nearly crumbled to the mat. He dipped as his legs badly buckled, and he tried to hold onto Mosley, who slipped away and bounced another right hand off Mayweather's head.

"I knew he was hurt [from the right hand earlier in the round], and I hit with the roundhouse right hand, and I thought he was going to go down the second time," Mosley said. "He was really hurt the second time. He held on for dear life. I was trying to get him off of me because he was hurt bad. He was clinching me hard. He grabbed me hard, and I flung him away because he had no balance, so I could get the next shot off. He grabbed me again. I was trying to get him out of there, and I knew the bell was going to ring.

"I said to myself, 'OK, I'll get him next round.' But he moved around, cleared his head and did what champions are supposed to do, and he outboxed me from there on. I just couldn't get to him again, and he won the fight."

Mosley said Mayweather showed him a lot to stand up to those shots.

"I don't believe it was my best punch. I believe the first right hand, I threw it hard, but not as a knockout punch," he said. "It just caught him really, really flush. The second time I threw it a little harder, expecting to hurt him when it landed, and I did. He took the shot very, very well with no problems, and comparisons between me and McGregor is not even in the same stratosphere. Before my first loss I was 38-0 with 35 knockouts. I was a knockout artist."

Then Mosley critiqued McGregor's punching, having watched how he strikes in his UFC bouts.

"He punches at the guy, not through the guy," Mosley said. "It's not that he doesn't punch hard, but the MMA guys crumble. As fighters, we're used to being hit with correct punches. The way he throws punches, it's not correct. There's a lot of slapping. It's not the same thing being punched by a real boxer."

Like many, Mosley doesn't give McGregor a chance to win.

"I don't think that he has a prayer. His age is an advantage: He's 29, Floyd's 40, so maybe Floyd can't move that well or his timing isn't there [after a two-year retirement], but I don't really give him a prayer. He will expect Floyd not to punch as hard as he's going to punch, and when he runs at Floyd and gets tagged, he will think he's a heavyweight and say, What the hell is this? It's not that Floyd's a big puncher, but he can hit. And he knows how to throw his punches the right way. When Conor gets hit by Floyd, he will feel like he will have been hit by the hardest puncher he's ever been hit by in the UFC."

Mosley surmised that if Mayweather could withstand his heavy right hands that he'll be able to do the same against a McGregor he feels doesn't get full leverage on his shots -- if he can even land them at all.

"Floyd has a great chin and could weather the storm," Mosley said. "Somebody with not as good of a chin would have gone down with those shots I hit him with."

Mosley added that Mayweather's power should be taken seriously despite not having a big knockout percentage.

"Mayweather has respectable power. He could knock you out if you went in there careless," he said. "When you run at him, you run into a shot, and it's like a collision. If McGregor is going to run in, he's going to get knocked out in, like, the fourth, fifth or sixth round. Conor is going to be tired. It's gonna be a whole different ballgame than the UFC. He won't even understand why he's in the ring. Mayweather has great defense, speed and underrated power."

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