Floyd Mayweather Jr. says it was the first punch of the fight. His promoter thinks it came during the second round.
One shot to the head, and suddenly Manny Pacquiao decided his night might better be spent fighting on the outside.
"I let him know it's not what you think it is," Mayweather said. "You're not just going to run in there. He felt a strength he didn't expect."
The knock on Mayweather — and the reason some people give Conor McGregor a chance Saturday night — is that he doesn't knock out people. Pacquiao never went down either, but Mayweather showed there are other ways to change a fight than putting a fighter on the canvas.
McGregor may find that out early if he goes after Mayweather at the opening bell as expected. The prevailing theory is that McGregor wins the power battle, but there's more than one fighter with power in this, the most unusual of fights.
"When he gets hit he's going to find out it's totally different," Mayweather said. "The brittle hands, they keep saying that, but when I come across his head he'll find out that it's totally different."
McGregor doesn't believe that, of course, and predicted again Wednesday that the fight would end early with Mayweather on his back.
"I believe one or two rounds, with 8-ounce gloves I don't see him surviving," McGregor said. "I'm starting to think I'll toy with him once he goes down."
The two fighters got together for the final pre-fight press conference to further hype a fight built on hype. In contrast to the bombastic tone of earlier appearances, it was a relatively subdued affair at the MGM Grand.
They posed together and didn't come close to exchanging blows. They did both manage to get in a few final words, though.
"It's not going to be easy Conor," Mayweather told his opponent. "I've got a great chin but remember this: The same way you give it you gotta be able to take it."
Though ticket sales have been tepid — largely because of astronomical prices — the pay-per-view is expected to be watched by some 50 million people in the United States alone and millions more worldwide.
It's half-fight, half-spectacle, a bout that matches a UFC star who has never boxed against a masterful ring technician with a record of 49-0. Born of internet hype, it has captured the curiosity of even those outside both sports and could end up being the richest fight ever.
And it's a bout Mayweather says he is taking very seriously, despite training at odd hours and holding meet-and-greets every night before the fight with fans who come to his strip club.
"I'm taking a gamble," said Mayweather, who is coming off a two-year layoff. "But it's worth it."
Mayweather's fight with Pacquiao ended up going 12 long rounds and turning a lot of people off from boxing. But while Mayweather hasn't really knocked out anyone in a decade, he says the streak will come to an end in what he insists is his final fight.
Oddsmakers in this gambling town hope it's not McGregor stopping Mayweather early as he claims. McGregor fans have placed thousands of small bets at long-shot odds of their man knocking Mayweather out in the early rounds, and sports books could suffer their worst loss ever in what is expected to be the most heavily fight ever.
McGregor also believes that's exactly what is going to happen. He said his years in the octagon and his varied skillset will translate well into the boxing ring, especially after a three-month camp where he used a former referee to help him adjust to boxing rules.
What he didn't bring in was any boxing trainer to teach him more about the sport, insisting that the team he has always been with knows him so well that it wouldn't have made a difference.
"We added what is useful and discarded what is not," he said. "As the final sparring session approached all the unnecessary was gone. It was a cleaner product, just the perfect product."
He was dismissive of Mayweather to the end, telling those attending the final press conference that Mayweather is not only a one-dimensional fighter but more than likely not the sports bettor he claims to be when posting his winning tickets online.
Perhaps, he said, that's the reason Mayweather still owes the IRS $22.2 million in back taxes.
"I think he has a big-time gambling problem," McGregor said. "He always shows his winning bets but never his losers. Maybe that's why he's in the position he is and had to take this fight."