World number one Novak Djokovic Saturday 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 next 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 last 30 Grand Slams.
But the Serb said Nadal's extended absence with knee problems proved that tennis remained unpredictable -- and that there were 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 from 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 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 favourite 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 win 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 favourite of the four majors, despite its notorious heat and reputation for surprises.
Last year, Djokovic outlasted Nadal over five hours and 53 minutes, a record for a Grand Slam final, to lift the title.
"Yes, the Australian summer can be brutal sometimes with the heat. But it is the way it is for so many years. It's not something that is a case from recent years," he said.
"I got used to it. I know how it feels like to practice, to play in the heat. You've got to go with the flow."