More money will have been bet on the Floyd Mayweather-Conor McGregor fight than on any other boxing match in history by the time the opening bell rings Saturday night.

It already is the most heavily bet fight ever at some sportsbooks, from Nevada to the United Kingdom.

On Thursday night, roughly 48 hours before the fight, William Hill's sportsbooks in Nevada had already taken more money on Mayweather-McGregor than they did on the 2015 Mayweather bout with Manny Pacquiao, which was the previous most heavily bet fight.

Nevada Gaming Control does not track the betting on individual boxing matches and lumps boxing into an "other" category, along with UFC, hockey, golf, tennis, auto racing and soccer, on the monthly revenue reports.

In May 2015, a record $81.1 million was bet on the "other" category, and bookmakers estimated the Mayweather-Pacquiao fight accounted for $60 million statewide. They're expecting the handle on Saturday's fight to easily eclipse that amount, with estimates as high as $80 million.

"It's going to be the biggest fight handle ever. Period," William Hill director of trading Nick Bogdanovich said.

On Thursday, two $1 million bets were placed on Mayweather. That's one more million-dollar bet than was reportedly taken on the Super Bowl earlier this year at Nevada books.

The Westgate SuperBook had already taken more six-figure bets on the fight than it did on the Super Bowl, according to vice president of race and sports Jay Kornegay.

On Friday morning, Mayweather's odds could be anywhere from -500 to -600 in Las Vegas. At sportsbooks in the U.K., he was down to -450. McGregor was around a 4-1 underdog.

The number of bets on just McGregor was rapidly approaching 10,000 on Friday at William Hill and was on pace to surpass the total number of straight bets (point spread and money line) on the college football national championship game between Alabama and Clemson.

And a large chunk of the action on the fight is still to come.

"Most people wait until the day of the fight to bet," said Ed Malinowski, sportsbook director at the Stratosphere.

By Saturday night, there may be more bets on just McGregor than there were on the New England Patriots in the Super Bowl at some books.

"I'm not sure I've ... seen this kind of lopsided action [in terms of the bets]," Kornegay, a 25-year veteran book manager in Las Vegas, told ESPN on Thursday.

Approximately 92 percent of the bets were on McGregor at the Westgate, but 77 percent of the money was on Mayweather. That's just on the odds to win the fight. There are dozens of proposition wagers available at the sportsbooks.

The worst-case scenario for MGM's sportsbook would be a McGregor knockout in the first round. The odds on a McGregor first-round knockout opened at around 45-1 and had been bet down to 15-1 as of Thursday.

Saturday's fight is setting betting records around the globe. Irish sportsbook Paddy Power was anticipating $250 million would be bet on the fight industrywide in the U.K.

On Friday, at Betfair, the U.K.'s leading sports betting exchange, £29 million ($37.3 million) had already been traded on Mayweather-McGregor. That's already nearly four times as much as what was traded on Mayweather-Pacquiao, the previous largest boxing market at Betfair.

The most heavily traded soccer match ever at Betfair, the 2010 World Cup Final between Spain and the Netherlands, generated £68 million in trades.

Australian bookmaker Tabcorp is also anticipating Mayweather-McGregor to be the book's most heavily bet fight in the company's 20-year history, topping the $4 million that was bet on a 2006 bout between Australians Anthony Mundine and Danny Green.

"To show the scale of interest, the Super Bowl held $1.5 million and Game 5 of the NBA Finals between the Cavs and Warriors held $1 million," Tabcorp's Matt Jenkins told ESPN in an email.

Three days before fight night, Paddy Power, known for its marketing savvy, went ahead and paid out on all Mayweather bets, $320,000 worth of them.

"Patriotism aside, we are paying out early on a Mayweather victory -- because we checked, and only one of them is a boxer," the sportsbook quipped in a release.

Las Vegas sportsbooks were expecting more large bets to show up on Friday and Saturday. The MGM sportsbook at the Bellagio on Thursday took a $1 million bet on Mayweather from a customer who wired in the cash to the cashier's cage. The deadline for such wire transfers was approaching Friday afternoon at the MGM.

"There's always been those times that guys will show up at the counter with a couple bags of cash," MGM vice president of race and sports Jay Rood said. "So that's in the realm of possibilities."

One potential large wager could come from Mayweather himself, who's known to regularly place five- and six-figure sports bets. He told the Los Angeles Times this week that he was planning to bet on himself with the largest wager he's ever made.

"You guys will see the betting ticket, don't worry," Mayweather told the paper.