As ex-Manchester United star Rio Ferdinand embarks on a career in professional boxing... what happened to Conor McGregor and other sports stars who entered the squared circle?

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Just over two years on from announcing his retirement from football, former England captain Rio Ferdinand has launched a sensational bid to become a professional boxer.

The former England captain, 38, will take on Betfair's 'Defender to Contender challenge' in the hope of earning a fighter's licence from the British Boxing Board of Control. 

But Ferdinand is not the first sportsman — or even the first former footballer — to turn his hand to boxing. Here, Sportsmail charts the fortunes of some of those who have tried to forge a career in the hardest game...

Former Manchester United defender Rio Ferdinand is set to become a professional boxer

The former England international has signed up to Betfair's 'Defender to Contender' challenge

Andrew 'Freddie' Flintoff

Flintoff, one of England's finest ever all-rounders, made his name as a destructive fast bowler and middle-order batsman.

The 6ft 4ins cricketer tried to take those skills with him into the ring when he began boxing under the guidance of father-son duo, Barry and Shane McGuigan.

In November 2012 Flintoff made his professional debut, topping the bill against unbeaten American heavyweight Richard Dawson at the Manchester Arena. 

Andrew 'Freddie' Flintoff made his name in cricket as one of England's finest ever all-rounders

Flintoff climbed off the canvas to beat American Richard Dawson on points in Manchester

Despite being floored in the second round, the ungainly Flintoff battled to a points victory in a fight labelled a 'circus' and 'disgrace' by some within the spot. 

The former cricketer never fought again but he defended his foray into boxing, saying: 'Some of the criticism I can understand, people want to respect this sport, that's fine. But some of the cheap swipes I can't be bothered with. 

'This was never about degrading boxing. We wanted to do it properly and show boxing for the fantastic sport it is. At times boxers don't get the credit they deserve.' 

Curtis Woodhouse

After former Sheffield United and Birmingham star Curtis Woodhouse fell out of love with football, he decided to pursue his 'first love' - boxing.  

Having sparred in the gym after training without his manager's knowledge, Woodhouse quit football altogether and made his ring debut in 2006.

Eight years and 26 fights later the 37-year-old achieved his dream by outpointing Darren Hamilton to win the British super lightweight title.

Curtis Woodhouse gave up a career as a professional footballer after growing to hate the sport

In the ring, Woodhouse went on to win the British title. He returned to the ring earlier this year

He initially retired with a 22-7 record after losing the belt to Willie Limond four months later. But having blown up in weight and seen his fitness deteriorate, Woodhouse got himself back in shape and returned to the ring earlier this month, outpointing Arvydas Trizno in Doncaster.

On Ferdinand's career change, he said: 'The training and everything didn't take me by surprise but I found learning the technical side of the game really difficult.

'There's a lot of things go on in a boxing ring that you don't realise until you get in there. It takes a long, long time to feel comfortable in the boxing ring.

'Your ego will take a bit of a knock. He's going to have to get used to a few setbacks along the way. He'll definitely struggle with the technical side of the game.' 

Anthony Mundine

The son of legendary fighter Tony Mundine and a talented basketball player, Anthony Mundine is one of Australia's most successful — and most controversial — multi-sport stars.

But it was in rugby league that he originally made his name. Having made his professional debut at 18, he was poised to become the NRL's biggest earner when he gave up the game altogether at 25.

With concerns over racism in rugby, the Muhammad-Ali-inspired Mundine took up boxing and made his debut in 2000.

Anthony Mundine was poised to be the highest paid player in the NRL before switching sports

The Australian (left) then went on to win world titles and beat the likes of 'Sugar' Shane Mosley

He was a national champion within five fights and challenging for a world title within 11 contests. He was beaten by Sven Ottke to the IBF super middleweight crown in 2001 but went on to win the WBA strap two years later.

Mundine then won the IBO middleweight title and recorded notable wins over the likes of Daniel Geale, Joshua Clottey and three-weight king 'Sugar' Shane Mosley.

Now 42, 'The Man' last fought in February and has won 47 of his 55 professional contests.

Sonny Bill Williams 

All Blacks centre Sonny Bill Williams is no stranger to a change in sport.

The 6ft 3ins powerhouse has represented his country at both rugby union and rugby league, with his combination of power and deft hand-skills making him a world star in both codes.

Unsatisfied with excelling in two sports, however, Williams has also forged a burgeoning boxing career alongside his exploits on the rugby field.

Sonny Bill Williams has represented New Zealand in both rugby union and rugby league

Alongside his career in rugby, the 32-year-old has forged a burgeoning career in boxing

In 2009, on the undercard of his good friend Mundine's world title clash with compatriot Daniel Geale, Williams made his debut against Garry Gurr, winning by TKO.

Over the next four years he would win five more fights and pick up the New Zealand and WBA international heavyweight titles. 

Unlike Ferdinand and many other cross-code stars, however, Williams has fought while still playing other sports. And having put his ring career on hold for nearly two years, the 32-year-old picked up his seventh straight win against Chauncy Welliver in January 2015. 

Leon McKenzie 

As the son of a former European champion and the nephew of ex-three weight world champion, Duke, it is perhaps unsurprising that Leon McKenzie chose a career in the ring.

But it was only after 18 years as a professional footballer that, at 35, the former Crystal Palace and Norwich star made his boxing debut.

The southpaw won all but one his first nine professional fights before losing an English super middleweight title clash with Jahmaine Smyle in November 2016.

Leon McKenzie was a professional footballer for 18 years, playing for sides such as Norwich

The southpaw then went on to win eight of his first nine fights before retiring earlier this month

After 10 months out of the ring, he returned to action earlier this month against Cello Renda. But McKenzie failed to pick up the vacant Southern Area title, losing by ninth-round stoppage.

The 39-year-old retired following the defeat, with his professional record standing at 8-2-1. 

'As much as my mind and heart want to continue boxing, unfortunately my body won't allow me, which leaves me no choice but to retire,' he said.

Conor McGregor 

The Irish UFC star swapped the Octagon for the squared circle earlier this year for a historic cross-over bout against Floyd 'Money' Mayweather.

In one of the richest fights in the sports history, the Irishman made his professional debut against one of the finest boxers of all time – who came out of retirement for the Las Vegas showdown.

McGregor had made his name as a destructive puncher in the UFC. But against an undefeated former five-weight world champion, few gave him even the slightest hope of victory.

Conor McGregor (left) became the biggest star in mixed-martial arts before turning to boxing

The Irishman was beaten by Floyd Mayweather in a money-spinning crossover bout in August

The 12-round showdown, which broke UK pay-per-view records, had been labelled a 'mismatch' and a 'joke' beforehand. But McGregor won a couple of early rounds before fatiguing badly and being stopped in the 10th round.

The Irishman is expected to return to MMA in the future but he has been called out by former sparring partner Paulie Malignaggi to fight on St Patrick's Day next year.

'This was some buzz to come in and face this man,' he said. 'So many doubters. First it was this fight is never going to happen, then it was like "I'm not going to lay a glove on him". So many things to overcome. I just enjoyed the whole experience.'