Teddy Atlas doesn't believe that we're going to see the "vintage Mayweather" in the ring, but admits he probably doesn't need to be to beat Conor McGregor. While Chael Sonnen demonstrates what McGregor has to do to avoid Mayweather's left hook. (1:49)

LAS VEGAS -- The fight most believed was nothing more than an impossible fantasy is happening.

The undefeated all-time pound-for-pound great Floyd Mayweather, he of 12 world titles in five weight classes, will exit a two-year retirement for one fight to face UFC star Conor McGregor, who is crossing over to boxing from MMA for a fight that was initially just a germ in his mind when he brought up the possibility in a 2015 television interview.

Two years later, huge amounts of social media fervor, genuine demand from combat sports fans intrigued by this pairing of huge personalities, a boatload of money and a wild four-city media tour ... and here we are.

Floyd Mayweather-Conor McGregor TV lineup for the Mayweather Promotions card on Saturday (Showtime PPV, 9 p.m. ET) from the T-Mobile Arena in Las Vegas: •Junior middleweights: Floyd Mayweather (49-0, 26 KOs) vs. Conor McGregor (0-0), 12 rounds •Junior lightweights: Gervonta Davis (18-0, 17 KOs) vs. Francisco Fonseca (19-0-1, 13 KOs), 12 rounds, for Davis' world title •Light heavyweights: Nathan Cleverly (30-3, 16 KOs) vs. Badou Jack (20-1-3, 12 KOs), 12 rounds, for Cleverly's world title •Cruiserweights: Andrew Tabiti (14-0, 12 KOs) vs. Steve Cunningham (29-8-1, 13 KOs), 10 rounds Preliminary bouts on Fox (7 p.m. ET): •Welterweights: Thomas Dulorme (24-2, 18 KOs) vs. Yordenis Ugas (19-3, 9 KOs), 10 rounds •Junior welterweights: Juan Heraldez (12-0, 8 KOs) vs. Jose Miguel Borrego, 10 rounds Fight facts Referee: Robert Byrd

Judges: Burt Clements, Dave Moretti, Guido Cavalleri

Start time: While the PPV begins at 9 p.m. ET, the main event won't begin before 11 p.m. ET and could start as late as around midnight ET.

"This was something that the fans wanted to see," Mayweather said. "I spoke to my team and said that the Mayweather-McGregor fight can be the biggest fight in history. That's what we're here to turn into reality."

But be honest, the fight makes no logical sense. Mayweather is a legendary boxer and supreme defender who will be facing an opponent who is outstanding in the Octagon but a novice in the ring and making his pro boxing debut.

Most experts view it as an abject mismatch, so that it was sanctioned by the Nevada State Athletic Commission was something of an upset. But it was, and now they'll engage in a 12-round junior middleweight spectacle that has the world's attention on Saturday night.

"Mayweather versus McGregor has captured the imagination of sports fans across the world. It's a historic event demanded by the fans and put on for the fans," Mayweather Promotions CEO Leonard Ellerbe said. "The fighters will put their talents on display in a fight that people did not think was possible. After it's done, people will talk about this fight for years to come."

This is your ESPN.com Ringside Seat for the fight:

Conor McGregor and Floyd Mayweather square off during a news conference at the MGM Grand in Las Vegas Stephen McCarthy/Sportsfile/Getty Images

Floyd Mayweather (49-0, 26 KOs) vs. Conor McGregor (0-0)

What's at stake?

Nothing and everything.

In the scheme of championship boxing, there are no authentic titles at stake, no pound-for-pound implications. Just a fight, but a big one that is one of the most hyped ever.

No matter how small the risk is for Mayweather -- many believe it will be one of his easiest fights -- there is still some element of risk when another man is throwing punches at your head.

Mayweather is 40. Mayweather hasn't fought in two years. And Mayweather, of Las Vegas, is gambling his perfect record and a brilliant Hall of Fame legacy that could be ruined with one punch from a guy who looks like he barely knows how to throw a proper boxing punch.

If McGregor, 29, of Ireland, wins, many will consider it the biggest upset in sports history. At least when Buster Douglas knocked out Mike Tyson it was a boxer knocking out a boxer. When the 1980 U.S. Olympic hockey team upset the Russians they were still hockey players facing other hockey players. When the Jets beat the Colts in Super Bowl III, the Jets were football players beating other football players.

McGregor is not a boxer, and there is no way he could make himself into an elite one in a matter of months. Mayweather, who comes from a boxing family, has boxed for virtually his entire life.

That experts give McGregor almost no chance to win, or to even compete, has merely motivated the "Notorious" one.

"It's certainly motivating," McGregor said.

"The disrespect and the disregard to my skill set is disappointing. I look at people so many times and their mind is closed. They've got a closed mind to how things can be done. It's a set way and there's no other way. Where if that was the case we'd never have reached across the waters and searched for other lands and we'd never have went into space." Conor McGregor

UFC president Dana White, McGregor's promoter, has been emphatic that his star has what it takes to get the job done.

"Don't judge a fight until the fight is over," White said. "You never know what's going to happen, especially with a guy like Conor who hits like a truck. When he hurts someone, he puts them away. Floyd is almost 41 years old. Conor is a young southpaw who hits hard. We'll see how it plays out. It's crazy to think that Conor won't connect with him."

Mayweather has played up McGregor's chances by pounding home the fact that even though McGregor is 21-3 with 18 knockouts in MMA, he has never lost standing up, only by submission.

"McGregor is a tough competitor. He's undefeated standing up. He's never lost when striking. I know that I'm in for a tough fight," Mayweather said. "But there is one thing I do know, this fight is not going the distance. No matter what anyone says, it's not going the distance."

For his part, McGregor is undaunted by his lack of boxing experience. He says his southpaw stance, which occasionally has given Mayweather minor problems, and his determination, aggression and punching power, especially in his left hand, will result in an early knockout.

"If people are overlooking my size, power and youth, they shouldn't be. I'm very confident in those attributes. I believe I'll knock him out early," McGregor said. "If you look at the way I fight, I am a fast starter. I come out and bang people right away. With eight-ounce gloves, I don't see him lasting more than one or two rounds. I am my own man and I feel I am superior to Floyd. When it comes to Saturday night, he will not be prepared for me.

"He's a beaten man. He will not be able to take the ferociousness that I come with. I'm very confident. I have more skills in many areas, and as a true martial artist, I can adapt to any situation."

The World Boxing Council will award the "Money Belt" to the winner of Saturday's fight between Floyd Mayweather and Conor McGregor. World Boxing Council

Cha-ching!

Many expect the fight to break all of the revenue records set by the 2015 megafight between Mayweather and Manny Pacquiao, including total revenue (about $600 million), most pay-per-view sales (4.6 million) and all-time gate ($72.2 million). The fight is also expected to set the Las Vegas fight wager record. Bookmakers handled about $50 million in action on Mayweather-Pacquiao and expect Saturday's fight to exceed $60 million.

"The numbers are tracking huge," White said. "We are going to do the numbers that everybody has been talking we are going to do. The over/under line in Vegas is 4.9 million (pay-per-view buys). Everything is tracking right now to say that we are going to kill this thing. So it is looking really good."

The fight will be available in more than 200 countries and territories, and because of the massive amount of revenue that will roll in, the fighters will each make nine-figure paydays. Mayweather, who has made more money than any other fighter in history, will add substantially to his earnings and is likely to surpass the roughly $250 million he earned against Pacquiao. McGregor's likely haul of more than $100 million will dwarf anything he has made for a fight in the Octagon.

"This is huge, the numbers that we've been hearing about, they've been crazy," Mayweather said.

Said Showtime Sports boss Stephen Espinoza: "This will be the most widely distributed event in pay-per-view history. This event will be distributed in over 200 countries [and territories] and might grow to 225 by the time we're done. We are well on our way to being a record-setting event in many categories."

McGregor is also thrilled with the business of the fight.

"I saw it pointing to record-breaking numbers all across the board, from the gate to the sponsorships to the pay-per-view. Absolutely every angle," McGregor said. "So I am very, very happy with how it has gone."

UFC star Conor McGregor will step into the boxing ring for his first professional bout and face Floyd Mayweather on Saturday in Las Vegas. Ethan Miller/Getty Images

Boxing vs. MMA

Of course, the fight is a boxing match not an MMA bout. But that hasn't stopped legions from looking at it as one sport against the other. It's silly, because they are different sports, but that's just the way it is.

McGregor really has nothing lose in that respect. If he loses, he was supposed to. Remember, he's not a boxer. A Mayweather loss is on him, but it would probably reflect badly on boxing because of Mayweather's stature.

"I truly believe I'm taking the bigger risk. But it's a big reward for both. I have the 49-0 record. When a fighter has lost before, if he loses again, they say, 'Oh, it's nothing. He's lost before.' But when a fighter has been dominating for 20-some years -- never lost -- everything is on the line; my legacy, my boxing record, everything is on the line." Floyd Mayweather

As long as it's a decent fight, White said, the outcome won't reflect badly on either sport.

"I don't think it hurts either sport," he said. "I think this is one of those cool situations where two guys are willing to take the risk to fight each other, and I think that this thing has captured the imagination of people. That is how this fight was built. This fight was really built through the fans and the media."

Mayweather, who has said he would consider himself a failure in the fight if he didn't score a knockout, guaranteed he will get it and that boxing has nothing to worry about.

"Nothing is a threat," he said. "Boxing is legendary, and boxing is here to stay."

He added, "When he gets hit, he's going to find out that this is totally different [than MMA]. The brittle hands? Let him keep saying all that. When I come across his head, he's going to find out what it really is. Believe me. This fight is not going the distance, I'm telling you."

McGregor is the outsider but hopes to prove his worth as a boxer.

"I get it. I'm coming in; I'm supposed to be from another sport and so again, I understand where it's at," he said. "But I'm just looking forward to [Saturday], going in and proving what I'm saying and then educating the world on what martial arts are and giving the fans and everybody a good solid fight and earning my respect in this game also."

ESPN Stats and Information

50-0?

When Mayweather retired in 2015 after waltzing past Andre Berto, he reached 49-0, the hallowed mark of the great heavyweight champion Rocky Marciano. Although it's not an actual record in boxing -- several fighters have begun their careers with more consecutive victories -- it is a revered accomplishment.

It seemed inevitable that Mayweather would return for a crack at 50-0, and here it is.

Mayweather, however, said that sweet round number is not a big part of his motivation for returning and he does not burn to surpass Marciano.

"I don't focus on other fighters, but I'm appreciative for every fighter that paved the way for me to be where I'm at," he said. "I always talk about how I appreciate the legendary fighters. My focus is to go out there and give the fans an exciting fight even though I know this is No. 50 and we're constantly talking about Rocky Marciano.

"I just always like to let the fans and the people know Rocky Marciano is a legend. Rocky Marciano did it his way, and I would just like to try to do it the Mayweather way. It wouldn't matter if I was 46-0, it didn't matter if I was 10-0 or 1-0. I just feel like I am blessed to be where I am at and I am appreciative."

Keep it clean

One question asked often during the lead-up to the fight is what would happen if McGregor decided to use his MMA repertoire on Mayweather -- maybe throw an elbow or kick or slam him to the mat.

Anything, of course, can happen, and that is probably one of the allures of the match for some. However, McGregor is not known as a dirty MMA fighter and everyone involved expects him to act professionally in the ring. Besides, if he doesn't, there will be big problems.

"There is a significant penalty. The details will remain confidential, but there is a significant penalty in the event of what could be described as an extreme foul, an out of the ordinary foul of what your normal boxing foul would be," Espinoza said.

White was adamant that it would not happen, saying not only would McGregor face heavy contractual penalties but also a lawsuit.

"That will not happen. First of all, obviously, that is absolutely in the contract, No. 1. And, No. 2, this is a boxing match under the Nevada State Athletic Commission under the rules of boxing," White said. "When you go to court it's always about damages, and the damages on Mayweather if something like that happened would be massive. That can't happen. ... Forget about the contract. [Mayweather] would just sue him and he would have a lawsuit against him that would destroy Conor and Mayweather would win."

Rafael's prediction: Mayweather by seventh-round knockout.