Floyd Mayweather Jr. will retire a perfect 50-0 -- but hey, at least the newbie Conor McGregor made him work for it.

McGregor's first boxing match was a heckuva show -- the 29-year-old mixed martial-arts superstar acquitted himself beautifully on Saturday night in Las Vegas, hitting Mayweather far more than the infamously slippery champ is used to -- but one of the 40-year-old Mayweather's powerful right-hand jabs dazed the Irishman badly in the 9th round, and that was all it took.

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Mayweather pressed the attack straightaway in the 10th; McGregor, looking totally gassed, had nothing left, and as Floyd's punches began landing more fiercely, veteran referee Robert Byrd called it. Mayweather would win his last fight in a knockout.

"I owed [the fans] for the Pacquiauo fight," Mayweather said afterwards, acknowledging that his last bout, a unanimous decision win against Manny Pacquiao in 2015, was not much of a spectacle.

McGregor went toe-to-toe with Mayweather in early rounds, landing straight-ahead jabs, counterpunching well and overall being more active and aggressive than his veteran opponent. Mayweather kept ducking, weaving, and jumping back -- he's the most elusive boxer who ever lived, after all -- but he looked oddly flat-footed, clumsily bobbing and weaving instead of dancing, and it began to look like McGregor had a chance.

Conor McGregor opened the fight with flair and power.Credit: Ethan Miller/Getty Images

McGregor was admirably composed and prepared (though he kept throwing borderline sketchy "hammer" punches to the back of Mayweather's head as he dipped at the waist, a legal punch in MMA but a big no-no in boxing). And the southpaw McGregor kept his right hand up in Mayweather's face at all times, a tactic that clearly irritated Mayweather, who kept batting the hand away to get an angle.

But it was McGregor who was finding his way through Mayweather's infamous defenses, and by the 5th round, Mayweather looked frustrated. He kept turning his back on McGregor and bending down, stopping the action and, at one point, annoying the MMA fighter, who's used to being able to pounce on that sort of thing in the Octagon.

This wasn't gonna last.Credit: Christian Petersen/Getty Images

But just when it looked like a huge upset was possible, Mayweather abandoned his retreating style in the 6th and began to take control, splashing McGregor's front with crafty counters and combos.

Then came the 8th -- one of the sluggiest, most entertaining rounds of boxing that Floyd "Money" Mayweather has ever been a part of: McGregor stung Mayweather with a stiff jab early, but Mayweather came slashing back with vicious hooks and jabs. He'd left something in reserve, and had chosen his moment to go for the KO.

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Then in the opening seconds of the 9th, McGregor turned the tables with a devastating (and possibly too low) body shot that looked to hurt Mayweather, possibly badly. Instead he flurried back, including a mid-round right jab that stuck clean to the flat of McGregor's left cheek -- a perfect punch. And that would do it. With McGregor dazed and fading, Mayweather went in for the kill -- but the bell rang to end the 9th.

The 10th wouldn't last long. Mayweather went right to work, and after McGregor took three rapid, heavy hooks to the head, the ref called it.

Mayweather and McGregor met in the middle of the ring to congratulate each other on a great fight; though they exchanged nasty, problematic and personal insults in the runup, everyone could see that these two fighters knew they'd just put on a barn-burner. Mayweather won it, and McGregor made it interesting.

These guys had fun.Credit: Jeff Bottari/Zuffa LLC/Zuffa LLC via Getty Images

"He's a lot better than I thought he was," Mayweather said. "He used different angles. ... Our game plan was to take our time, go to him, let him shoot all his heavy shots early, then take him down during the stretch."

McGregor, ever the cocky showman, managed a backhanded compliment that, frankly, describes Mayweather's winning formula perfectly:

"He's not that fast, he's not that powerful, but he's composed," McGregor said. "I thought it was close, though!"

But closeness doesn't matter when it's time to count the money.

Mayweather came out of retirement for yet another $100 million guaranteed purse, and will make more than $200 million after his piece of the pay-per-view comes in from Showtime. McGregor's purse is $30 million -- and while that's significantly less than Mayweather, it's far more than he's ever made for a UFC fight. He was expected to walk away with something in excess of $100 million when it was all over.

And for Mayweather, it will be.

"This was my last fight," Mayweather said.

At 40 years old and a perfect 50-0, with one last great knockout to go out on, there's good reason to believe it. Mayweather may not be a good guy, but as fighters go, he'll go down as the greatest of a generation.

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