Will Nadal recover from knee tendonitis once again, or will the big four of tennis become a big three?
Men's tennis is experiencing a golden era. At the top of the game are four titans - Novak Djokovic, Roger Federer, Andy Murray and Rafael Nadal - who perform so consistently that it's almost the norm for them to occupy all semi-final berths of a tournament. When they do not, eyebrows are raised.
For a perspective on how challenging it is to break the dominance of the game's elite, consider that it took Murray five attempts to win a grand slam final. He wrestled the 2012 US Open away from the clutches of Djokovic, but only after an epic struggle. Djokovic found a champion's spirit when two sets behind and forced Murray to defy his nerves in a deciding fifth set.
Of the four, Djokovic was third to climb the ladder to tennis greatness. He did so by lifting the 2008 Australian Open title which presented him as a genuine rival to the then top two, Federer and Nadal. But Djokovic only continued to snap at their heels until 2011 when he dominated the calendar year, winning his first 41 matches and three of the four majors.
He has had a career-long rivalry with Andy Murray, who mirrored his progression through junior to senior ranks. However, Murray has had to endure many more painful defeats during his rise; he lost once to Djokovic and three times to Federer in grand slam finals before finally winning one of his own.
It is testament to the supremacy of the top four that some tennis circles contend that any one of them would be world number one in a different era. The performances in last year's Australian Open final from Djokovic and Nadal were seen as superhuman. The top four shared all four grand slams in 2012. Them aside, only Juan Martin Del Potro has won a major since Marat Safin won the Australian Open in 2005.
Things are likely to change in 2013. Age has been a recent threat to the longevity of Federer's career, but his play shows few signs of relenting. He has always found a way to adapt his game - by adding an extra few miles per hour to his serve, for example - and his simple yet slick technique continues to be devastating. Despite this, the gap between Federer's decline and the rise of Djokovic and Murray will inevitably increase.
The season-starting Australian Open is missing a big four name. Rafael Nadal has not played since losing surprisingly in the second round of Wimbledon and his absence leaves a major hole in the tournament. Nadal has suffered with knee tendonitis throughout his career, but for him to face another lay off of this magnitude is very worrying.
It is a chilling absence. Nadal is only 26 and has been faced with career-threatening injuries at the regularity that most players suffer mere niggles. His high-intensity style lends to heavy impacts on his joints and Nadal has paid the price of his successes. At the highest level, tennis is about building up momentum and maintaining steady form throughout a major, and there is a fine line between winner and runner-up. The spectacular 2008 Wimbledon final ended tamely when Federer hit a routine forehand into the net with both players competing at the brink of exhaustion.
Whether Nadal can continually return from an injury as serious as knee tendonitis to the pinnacle of the game is uncertain. It is a recurring problem that he will face throughout his career and while Nadal is getting his knees back to working order, Murray and Djokovic will be fine tuning, tweaking and honing their arsenal. It puts him at an obvious disadvantage.
Murray has proven he is now one of the very best and barring the extraordinary 2013 will see him battle with Djokovic for the game's highest honours. Federer will be there or thereabouts, as there are no indications to say otherwise. The question mark looms over Nadal and whether he is able to recover from his latest bout of injury.