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Those of us who had the fortune of watching last year's epic Wimbledon men's final - probably the greatest of all time - can breathe easier today, with the news that Rafael Nadal will be defending his crown. The world number one successfully came through a three-hour practice session on Tuesday, and will now play in the Wimbledon warm-up exhibition event at the Hurlingham club in a bid to get some much needed grass court practice before he opens centre court proceedings on Monday.

The Spaniard ([5.9] to win Wimbledon) pulled out of last week's AEGON Championships, won by Andy Murray, with a recurrence of the knee injury that has flared up over the last few years. It's a problem that has many experts fearing that Nadal's career will be curtailed long before it should be, thus depriving the sport of someone who is already a legend of the game.

Nadal missed last year's Davis Cup final because of the same problem, and has admitted to being emotionally devastated when a doctor suggested he might never be able to play tennis again. Fortunately, that diagnosis was overly pessimistic, but it does appear that Nadal has a serious problem that could affect his longevity. The immediate question though is: what are the chances of him performing a successful title defence?

Nadal's game is based on his physical and mental strength. He has been the iron man of tennis in recent years - able to run faster and hit harder for longer periods of time than anyone else on tour - and that has given him the mental edge over his peers. He burst onto the Grand Slam winners' circle in 2005, taking the French Open title at his first attempt wearing his sleeveless shirt and three-quarter length trousers that made him look like a modern day goliath despite his tender years. However, this year, Nadal's look has been somewhat different. Gone are the sleeveless tops, replaced by a more classic outfit that renders him less intimidating on court, which some have written off as no more than an illusion.

The truth, however, is that the sartorial change was in all probability a calculated move that was somewhat forced upon him, because of his suspect knees. Nadal was advised by doctors that he needed to reduce his overall weight, to reduce the pressure it was putting on that part of his body, if he wanted to minimise the risk of severe injury. His new outfit could well be to disguise the fact that he is not quite the muscular physical specimen of previous years.

The decision taken by the Nadal camp to reduce his overall muscle mass for the sake of his knees has two effects. Firstly, Nadal does not have quite the same weight of shot off the ground, and secondly he does not have the same incredible powers of recovery that have stood him in such good stead during each of his Grand Slam wins. While these effects are only fractional, at the very top level the smallest differences can be the difference between winning and losing.

Furthermore, as players cotton on to the fact that Nadal isn't actually the superhuman behemoth of 2008, his ability to intimidate opponents lessens slightly, meaning they aren't beaten before they even step on court as they may previously have been. His aura of invincibility hasn't been helped by his French Open defeat to Robin Soderling [85.0], where he was considered to be untouchable. If a journeyman can do that to him at Roland Garros, there must be several more players who would like a crack on the lawns of SW19.

No player wants to perform at less than 100 per cent fitness although it is commonplace on the circuit, partly because of the nagging doubt it leaves in the back of their minds, but in Nadal's case it's exacerbated even further. He plays such a physical game, scrambling for every ball, ripping winners at full stretch, that if there is even the slightest doubt in his mind about whether his body can hold up, he will be less than fully committed to the balls that are just out of reach. The result? Over pressing too early in the rally and dulling the strengths that set him apart from the pack. That's why legends of the game like Bjorn Borg have said they don't believe that Nadal will successfully defend his crown.

Nadal will be hoping for a kind draw to ease him into the tournament, and give him some much needed match practice, because he will certainly be vulnerable in the early rounds. He will be hoping not to face players who can simply hit through him in the same fashion that Soderling did in Paris, and that type of opponent is becoming increasingly commonplace. If he does survive into the second week though, there are several power players who would fancy their chances. The likes of Jo Wilfried Tsonga [44.0], who decimated Nadal in the Australia Open in 2008; James Blake [350.0] or Fernando Gonzalez [380.0] could simply blow him away, not to mention his peers in the 'Big Four'.

As it stands, Nadal is third favourite behind Roger Federer [2.28] and Andy Murray [4.9], although expect that to drop if he gets into his stride in the first week. But even if he does make it into the second week with the minimum of fuss, he's going to face at least one opponent who will go on court believing he is going to beat the world number one.

In short, if Nadal does retain his trophy, it will be the greatest win of his career, and will force many seasoned experts to eat their words.