One belts out hits for a living, the other belts opponents in the boxing ring. But this hasn't stopped the unlikely pairing of singer Robbie Williams and heavyweight boxer Tyson Fury from releasing a Christmas duet.
Their song, Bad Sharon, urges listeners to "forget everything that went wrong and all sing along" before it becomes a bawdy anthem about a Christmas party. Williams and Fury entreat listeners to put on their glad rags, drink shots and "grab that Sharon from the office", presumably somewhere near the photocopier.
"If it catches on, it could be huge," says the Official Charts Company, which lists the single as a contender to be Christmas number one.
Bad Sharon is the latest in a long line of incongruous duets that stubbornly pop up every Christmas. A few of these have become classics: the Pogues and Kirsty MacColl's Fairytale of New York or David Bowie and Bing Crosby's Little Drummer Boy, for example. Some work due to the sheer unexpectedness of the pairing, such as crooner Tony Bennett's version of Winter Wonderland with avant-garde pop queen Lady Gaga.
But most of them – from Sir Terry Wogan's pairing with Aled Jones on 2009's Silver Bells, to Justin Bieber joining forces with rapper Busta Rhymes on 2011's Drummer Boy, to Messrs Williams and Fury – are the aural equivalent of the Christmas jumper: cheesy, gaudy and throwaway.
So why do people still buy them? Prof Darren Sproston, director of the School of Arts and Media at the University of Chester, says the phenomenon is related to the traditions of carolling and wassailing, and "society coming together as amateurs to participate in music-making".
It is only when he lists all the past Christmas number ones with "amateur" elements that the public's yuletide acceptance of the unpolished becomes clear. Many of the hits weren't even Christmas songs. Paul McCartney's Wings had the Christmas number one in 1977 with the pared-back Mull of Kintyre, which features Mull's local Campbeltown Pipe Band.
Two years later, Pink Floyd topped the chart with Another Brick in the Wall (Part 2), a song underpinned by a coarse children's choir singing, "We don't need no education." The following year St Winifred's School Choir took the honours with There's No One Quite Like Grandma. The list goes on, from Military Wives to the Lewisham and Greenwich NHS Choir.
Last year's number one was a poor rendition of Starship's We Built This City (in which the words "rock 'n' roll" were transposed with "sausage rolls") by YouTube star LadBaby. These songs are far from universally awful, but they all sound at least partially home-made.
Even Fairytale of New York, a duet often voted as the greatest Christmas pop song, is infused with a DIY spirit.
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"It has that folky element," says Prof Sproston. "Again, it is playing into amateur music-making. If you take the lyrics away, you can imagine it being played in an Irish pub quite easily." It is noteworthy that Williams praised Fury's voice for its "gravel, grit and personality" as much as its tunefulness.
Due to our obsession with ritual at Christmas, we have now come to expect such songs at the end of every year, adds Sproston. Habit has made them as big a part of the festive season as the Queen's Speech or brandy butter.
Still, there is "way more bad Christmas music than good", says Mitchell Kezin, an American filmmaker. Kezin's 2013 documentary Jingle Bell Rocks! features people, like him, who think nothing of spending eight hours in a dusty church basement digging through crates, looking for obscure and offbeat festive 45s. Songs mentioned in the film include Santa Came on a Nuclear Missile by the aptly named Heather Noel, Santa Claus is a Black Man by Teddy and Akim Vann, and Close Your Mouth (It's Christmas) by hippy group the Free Design.
But, Kezin says, people tend to be emotionally vulnerable at Christmas. "They think, 'Oh, I love that artist and it's a Christmas album. I'm going to buy it and love it and throw my critical listening skills out of the window.' "
The record companies, of course, know this and take full advantage. They also know that if they make their Christmas song a duet they can target not just one set of fans, but two.
Nevertheless, Kezin says, there are some crackers among the turkeys. His knowledge of notable bizarre duets wouldn't fit on this page. But Swedish rockers the Hives' collaboration with Eighties pop star Cyndi Lauper in 2008, A Christmas Duel, is both "funny and filthy", he says. "I bought no gift this year and I slept with your sister," bellows the Hives' Pelle Almqvist. "I bought no tree this year and I slept with your brother," Lauper retorts.
Meanwhile, country singers George Jones and Tammy Wynette's Christmas duets of 1973 rank among Kezin's favourites. The pair were not incongruous musically – both were country music royalty. Rather, it was their personal lives: they were in the middle of a stormy marriage, and their Christmas cheer is therefore seasoned with something "painful and raw", Kezin says. And then there's jazz trumpeter Miles Davis's song with Bob Dorough, best known for singing Three is the Magic Number. Blue Xmas shouldn't work but it does (although Kezin says it's technically "two people playing together" rather than a duet).
So what makes a good Christmas duet? Kezin says it must be heartfelt, meaningful and original – which wipes out half the songs ever released.
Perhaps we should add another key Christmas tenet: forgiveness. It's the season of goodwill, so it's time for us all – in the words of one of this year's would-be smash hits – to "forget everything that went wrong and all sing along".