Over the years, Floyd Mayweather Jr has won and lost millions gambling. He has decorated his adopted city of Las Vegas with the style of an old-time loudmouth, literally throwing his money away, across nightclub floors to easily dazzled sycophants, and in bets on everything from golf to American football. But the risk he is taking by agreeing to fight the undefeated young Mexican Saúl "Canelo" Alvarez in the second of his contracted six fights with Showtime at the MGM Grand in Las Vegas on 14 September might be the biggest of his career.

It is a calculation as precise as the right cross with which he tormented poor Robert Guerrero in his last fight, over 12 rounds in Las Vegas earlier this month, the first of the "Farewell Six", and I can't see Mayweather getting it wrong, although it should be a stirring encounter.

What fascinates is the timing. Mayweather knows he has to fight Alvarez at some point before he retires in two years' time, so he figures he should take the 22-year-old now before he gets even better than he is, and while he is still within Mayweather's weight range. The chunky, red-headed dynamo, who owns the WBC and WBA light-middleweight belts, has agreed to come down to a catchweight of 152lbs and says he is comfortable with that.

Leonard Ellerbe, who has never thrown a paid-for punch in his life, spoke with typical manager's gusto when he told the Las Vegas Review-Journal on Thursday : "Canelo [cinnamon in Spanish] can come in at 200lbs if he wants to, he'll still get his butt whipped. Floyd is the best athlete in sports. He won't have any problems with 152lbs… From day one, Floyd made it very clear he wanted to fight Canelo Alvarez. This is what the fans want. It's the biggest and best fight out there, and now it's going to happen."

At 36, Mayweather sees the finishing line far more clearly than he did while fiddling his way out of a commitment to meet a prime-time Manny Pacquiao in recent years, and that ship has sailed. If it ever returns, it will be an anti-climactic pension fight for both of them, and for that they deserve the opprobrium of the boxing community, as does the promoter Bob Arum, whose power struggle with Mayweather was at the core of the stalemate.

So, we move on. Alvarez, like Mayweather, is unbeaten, the one blemish on an eight-year CV his draw against one Jorge Juarez in Tijuana in 2006. Mr Juarez, who last fought two years ago, finished his career with 18 defeats in 29 outings and was knocked out seven times – which only goes to show what a funny business this can be.

This, though, is no laughing matter. And this will be no reprise of Mayweather's comeback win over Guerrero. That lop-sided, one-handed victory was a masterclass of pure boxing skill. Guerrero, dogged as he was, lacked the zip or the nous to unlock the Mayweather puzzle. Alvarez, despite his one-dimensional image, is not only a smarter, more accomplished fighter than the Californian, he is bigger, hungrier and more dangerous.

Nonetheless, I think Mayweather will extend his record to 45-0 and I think he will go one better than Larry Holmes years ago and eventually surpass the 49-0 mark of Rocky Marciano. Mayweather likes the look of 50-0 and sayonara. He is a perfectionist, an egotist and, unlike Holmes, in control of his destiny.

If you want to know what a control freak Mayweather is, he announced this fight on Twitter, and was joined by Alvarez in Spanish. But Money May did not need Alvarez's agreement, nor did he need the machinery of the public relations machine. He did not need the approval of his manager or his business partners at Showtime. Unprecedentedly in boxing, Mayweather is the No1 man in the business, from the ring to the boardroom.

If Alvarez thought he had any say in this, he is deluded. If he imagined he was going to bring heavyweight clout to the negotiations, he would have been disabused of that notion the moment his people sat down with Floyd's people. He is a fine champion, potentially a terrific one. But for now he is a dancing partner and should be grateful for the opportunity to cause an upset.

The consequences of Mayweather losing are immense. It would not only derail his grand plan, it would blow a hole in the strategy of Showtime, who lured him away from HBO where he has spent nearly his entire career.

Talking last week in London with HBO executives who had come over for Carl Froch's fight with Mikkel Kessler, it was apparent the switch was a blow. But it's business. HBO knew it might happen – and Floyd controlled every minute detail of it.

On 14 September, he will attempt to do the same to Alvarez. If the Mexican gives him as good a fight as he is capable of doing, he will lose nothing.

If Alvarez wins, he will instantly become the superstar some people think he already is. I don't see him winning, but I see him bringing out another late-career performance from Mayweather to go with so many great ones before.

After that?

Still in the queue is Amir Khan. He moves up to welterweight in December, possibly against Devon Alexander. If that happens, if he wins, he will still be lining up for his chance next May, or the following September. He might even queue-jump his old friend and gym mate, Pacquaio.

But whatever happens, he will be as grateful as Alvarez – and the call will be in the gift of one of boxing's most extraordinary characters.