World No 1 Novak Djokovic warned Rafael Nadal's injury woes showed the Grand Slam stranglehold exerted by the top men's tennis players was not unbreakable.

Djokovic starts his bid for a third straight Australian Open title this week, a feat which is unprecedented in the professional era and would extend a record in which the top four have won all but one of the past 30 Grand Slams.

But the Serb said Nadal's extended absence with knee problems proved that tennis remained unpredictable - and that there was a host of players waiting to take advantage if the top men falter.

"I think we cannot predict anything. It's individual sport, so it only depends on you. Anybody can have what Nadal has at this moment," Djokovic said in Melbourne.

"That's why you have to be so committed and professional for daily routines. Small details matter for the long term, the preparations you do, the practice, the recovery.

"If you are injured, you're off the tour. Your ranking is going down."

Argentina's Juan Martin del Potro is the only man besides Djokovic, Nadal, Roger Federer and Andy Murray to win a Grand Slam tournament since 2005.

Nadal, an 11-time Grand Slam winner, missed much of last season with ongoing knee problems and announced his withdrawal from the Australian Open last month citing a stomach bug.

"It's probably expected that the three of us, and Nadal, of course, would still be main candidates to win all the major titles," Djokovic said.

"But, you know, I wouldn't underestimate del Potro, (Jo-Wilfried) Tsonga, (David) Ferrer, (Tomas) Berdych, anybody who is in the top 10. I'm sure there's new young players coming up like (Bernard) Tomic, (Grigor) Dimitrov, (Milos) Raonic.

"I don't think it's nice for me to predict that us three will be champions of all Grand Slams this year."

Djokovic will start the tournament against France's Paul-Henri Mathieu as favorite for the trophy, which would take him level with Federer and Andre Agassi on four titles at Melbourne Park.

He said he wasn't sure why nobody has won three Australian Open titles in a row since Roy Emerson completed a string of five straight in 1967, two years before tennis embraced professionalism.

But the Serb, who has won three of his five major titles at Melbourne Park, including his debut Grand Slam win in 2008, said it remained his favorite of the four majors, despite its notorious heat and reputation for surprises.

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