Rafael Nadal's exit sparks doubts

World sport's greatest individual rivalry may be over, as Rafael Nadal's woes continue.

Nadal's Australian Open withdrawal has not only robbed the tournament of one of its major drawcards but also left the Spanish superstar's career in limbo.

Nadal's second consecutive grand slam scratching will end his seven-year reign in the world's top four, and his rankings slide will continue if he is unable to defend the points amassed during his most successful claycourt campaign ever last season.

With Novak Djokovic re-establishing himself as the game's dominant force, Andy Murray emerging as the world No.1's pre-eminent rival, Roger Federer in the twilight of his career and Nadal, 26, weak at the knees, the tennis landscape may have shifted forever.

No two men have slugged it out in more grand slam deciders than Nadal and Federer but, unless Nadal makes a complete recovery from tendonitis in his left knee, sports fans may never see the two titans clash again in major finals.

While Federer has vowed to continue playing until 2016 and remains No.2 in the world, the Swiss maestro turns 32 in August.

With Nadal set to lose his top-four status, he will no longer be guaranteed to avoid Djokovic, Federer and Murray until the semi-finals at the slams.

He has been sidelined for six months and is targeting a return at Acapulco on February 27, although the 11-time major winner hasn't ruled out playing at an earlier event, if his recovery goes well.

The Majorcan's latest setback stems from a virus he and his medical team feel has cruelled his chances of being competitive at the Open, which starts on January 14.

In a statement, Nadal said he wouldn't have the proper preparation to play in a best-of-five-set tournament straight away.

Nadal stressed his decision had nothing to do with the knee tendinitis that led to his hiatus since suffering a shock second-round loss to then 100th-ranked Lukas Rosol at Wimbledon in June.

''My knee is much better and the rehabilitation process has gone well, as predicted by the doctors,'' Nadal said. ''But this virus didn't allow me to practice this past week and therefore I am sorry to announce that I will not play in Doha and the Australian Open.''

Nadal's coach and uncle, Toni Nadal, said it was ''not conceivable'' for the former No.1 to make his comeback at Melbourne Park.

''He wouldn't be ready for that,'' he said.

Nadal's knee injury prevented him from defending his Olympic singles gold at the London Games, where he was supposed to be Spain's flag bearer in the opening ceremony.