A breakdown of the most memorable boxing and MMA fights featuring fighters crossing over from one sport to the other

The idea of mixed martial arts fighters and boxers competing to determine the superior sport has been around for decades and is something argued about fervently by fans of either sport. Every so often a boxer competes in MMA or vice-versa, and the fights very rarely live up to the hype.

The reason being that the vast majority of boxing vs. MMA fights are complete mismatches featuring usually washed up or semi-retired fighters moving sports on a one-time basis to make some cash before they retire altogether.

Few bother to put in the right kind of training and most know full well that despite talking a good game, they realistically have very little chance of emerging victorious.

The truth is that either sport takes years to become proficient in and, even taking a year away from competition to train in a new discipline exclusively, doesn't prepare for a fight against someone who may have been competing in the sport for 15-20 years.

Despite the quality or length of the fights being questionable, however, there are memorable fights every now and then either for their place in the history of each sport or simply because of the names associated.

Ray Mercer vs. Tim Sylvia

Former heavyweight champion Ray Mercer fought former UFC heavyweight champion, Tim Sylvia, in what most expected would be a mercifully short and brutal bout at Adrenaline MMA III: Bragging Rights in 2009.

The fight was certainly short but didn't go as any expected as the 48-year-old 'Merciless' sparked Sylvia with a single punch nine seconds into the first round.

Sylvia at the time had lost three bouts on the bounce but was still considered to be top ten in some circles having lost to elite competition.

The Mercer loss was something that Sylvia's career never fully recovered from and it was even rumoured that he was blackballed by the UFC after the loss.

The fight damaged the credibility of MMA in the eyes of some since a completely over the hill boxer knocking out a former UFC champ was how the headlines tended to read.

They didn't mention that Sylvia came into the bout well over 300 pounds and undoubtedly at his lowest career ebb.

Ricardo Mayorga vs. Wesley Tiffer

One of the less well-known bouts on the list, many may instead remember that Ricardo Mayorga was scheduled to fight former UFC lightweight contender Din Thomas in 2010 for Shine fights.

Mayorga's promoter at the time Don King eventually stopped the bout and likely saved Mayorga from a one-sided beating.

Mayorga's actual MMA debut came in 2013 following his initial retirement from boxing and saw the fight end with victory for the Nicaraguan.

The fight was later ruled a no contest after it emerged that Mayorga has ended the fight via an illegal knee to the spine of Tiffer.

Mayorga would go on to lose his next three MMA fights before retiring and eventually returning to boxing.

Randy Couture vs. James Toney

UFC 18 in January 2010 saw former two-weight UFC champion, Randy Couture, takedown and quickly submit former three weight boxing champion, James Toney, who entered the octagon after reportedly signing a multiple fight deal after meeting Dana White backstage after an event he attended.

Toney looked as if he had trained very little for the fight and seemed to lack even basic grappling skills.

Regardless of his training, however, Toney was entering the fight some 19 years after he won his first boxing world title, and at 42 was a long way from his physical peak.

Couture was even older at 47 but was something of an anomalous fighter in that he hadn't peaked until his late 30s.

The fight was even rumoured to have been made by the UFC in response to the Sylvia vs. Mercer, in an attempt to repair any damage done to the reputation of MMA.

Muhammad Ali vs. Antonio Inoki

Although largely lost to the annals of time, the greatest boxer of all timetook part in a mixed rules fight back in 1976 in Tokyo against professional wrestler and mixed martial artist Antonio Inoki.

The specific conditions of the fight were not clear nor was whether the fight was genuine or at least supposed to be.

Initially, it seems that the Ali camp were under the impression that the fight would be an exhibition.

Inoki, on the other hand, was expecting a full fight and, as a result, the Ali camp changed some of the rules in the days leading up to the bout, banning Inoki from landing proper kicks or using grappling.

This resulted in the very little action and an inconclusive end to the fight which was ruled a draw after 15 rounds.

Inoki spent the majority of the fight on his back kicking at Ali's legs and the heavyweight champion for his part threw the odd jab and kept his distance.

Floyd Mayweather vs. Conor McGregor?

If this fight comes off, and despite reports of progress it is still far from a done deal, it may break the current pay per view records set by Mayweather vs. Pacquiao of 4.6 million in 2015.

That alone should put this fight at the top of the list.

Conor McGregor is currently the biggest draw in MMA and the reigning UFC lightweight and former featherweight champion.

Floyd Mayweather, although technically retired, is still the biggest pay per view draw in boxing, and transcended the need for titles along time ago.

Although a boxing match between the pair would be another mismatch it would represent the first time a fighter in their prime made the switch to competing in a new sport at the risk of being soundly beaten.

The phenomenon of cross-discipline fights isn't something new and is essentially how MMA started out.

In the early UFC events, most fighters were trained in one discipline and were testing their skills against a fighter similarly limited to one set of techniques.

Modern MMA fighters against fighters trained only in striking aren't something new either, as numerous K-1 kickboxers have tested the waters in MMA over the years to mixed success.

Mirko Crocop was already a veteran of 50+ kickboxing fights before he debuted in Pride FC and went on to achieve a top three world ranking in MMA.

Elite kickboxing contemporaries such as Stephan Leko or Peter Aerts didn't fare so well, racking up one win between the pair with five losses.

MMA fighters have also shown interest in pro boxing although, similarly to most boxers who express an interest in changing sports, it usually never happens.

Former UFC middleweight king Anderson Silva was one such high profile fighter looking to switch and had even held talks with Roy Jones Jr although, in the end, nothing came of the idea.