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UFC star Conor McGregor has added more fuel to speculation surrounding him and a possible showdown against retired boxer Floyd Mayweather Jr.

The man known as The Notorious posted the following on his Twitter feed on Sunday:

On Saturday, Mayweather revealed he was the one who started the fight rumors (warning: NSFW language):

According to the Sun’s James Beal and Matt Heath-Smith, the two parties have been negotiating for a while and agreed to the fight earlier this month but still need to settle the purse and paperwork.

UFC President Dana White has denied the reports, though.

"It's not true; it's just a tabloid story," he told Yahoo Sports' Kevin Iole. "And as far as I knew, Floyd is retired, and he's been on a worldwide vacation and hasn't even been thinking of fighting."

Per the Sun report Friday, Mayweather is set to earn at least £100 million ($144.8 million), while McGregor will have to settle for £7 million ($10.1 million). The Sun quoted its source adding more details:

Floyd went mad after Conor said he would crush him and knock him out in 30 seconds, he’s obsessed with making Conor eat his words. Floyd is calling the shots. It’s strictly boxing and Conor will have to lose roughly 25lbs. It was agreed in principle on May 2, but Floyd wouldn’t reveal the identity of his opponent until last night when he told friends he’ll be fighting Conor.

However, Ariel Helwani of MMA Fighting reported there was "no truth" to the rumours about the fight.

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McGregor challenged Mayweather to a fight back in January, according to Rob Burnett of the Daily Mirror. Per Gareth A. Davies of BT Sport (h/t James Orr of the Independent), he repeated his challenge in March, saying he would be willing to meet the undefeated Mayweather in a ring.

"I’m open to discussions; come at me," he said. "But I hold the key, and it’s me who holds the key to the fight game.

"I am open-minded; I am open to discussion; I love competition; I love fighting. It would be nice to make that fight if the fans were interested and wanted to see it."

Mayweather retired after his unanimous-decision win over Andre Berto in September last year, but per ESPN.com’s Darren Rovell, his company recently filed trademarks for "TMT 50" and "TBE 50." The veteran’s record currently sits at 49-0, the same as former heavyweight champion Rocky Marciano.

With sole possession of the greatest undefeated record in boxing history within Mayweather's reach, most pundits assume the 39-year-old will return to the ring at some point. Per Showtime Sports, he recently revealed he has been talking to networks and could be open to a return:

Money said he’s looking for a bout worth "nine figures," and McGregor, who is one of the biggest stars in combat sports today, would likely be an opponent who could make such a megafight happen.

The Notorious recently retired and then un-retired from the UFC, and White made it clear the Irishman won’t be competing at UFC 200 on July 9, per the Los Angeles Times' Lance Pugmire. He was supposed to headline the event in a rematch with Nate Diaz, who beat McGregor at UFC 196 on March 5.

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That leaves the 27-year-old southpaw without a bout on his schedule, although he’s expected to defend his featherweight title against the winner of the fight between former champion Jose Aldo and Frankie Edgar at UFC 200, per MMA Fighting's Shaun Al-Shatti.

Odds Shark lists Mayweather as the early heavy favourite in a theoretical bout at minus-2,000 odds, while McGregor is listed as a plus-900 underdog.

While McGregor is known as an excellent stand-up fighter with superb hand speed and punching power, he would find himself out of his comfort zone against one of the greatest defensive fighters boxing has ever seen.

Money is almost unhittable and is a master of moving around the ring. Combined with his ability as a counterpuncher, he shouldn't be troubled by McGregor, should they ever meet.