MELBOURNE  - Novak Djokovic bids for an unprecedented third straight Australian Open title from Monday, with growing rival Andy Murray and the great Roger Federer shaping as his biggest challenges.

Argentina's Juan Martin Del Potro is the only man besides Djokovic, Rafael Nadal, Federer and Murray to win a Grand Slam tournament since 2005, but the Serb said there were plenty of others who could threaten their dominance. "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 face a stiff challenge from the newly assertive Murray, fresh from his first Grand Slam win and keen to avenge his final and semi-final defeats to Djokovic in Melbourne in 2011 and 2012. Also pressing hard will be Federer, holder of an unmatched 17 Grand Slams and still hungry for more even though, at the age of 31, he is conceding six years to his younger rivals.

Murray, the world number three, was thrashed by Djokovic in the 2011 Australian Open decider, and narrowly lost out in their thrilling five-set semi-final last year. But the Scot then beat Djokovic on his way to London Olympics gold, and again in the US Open final as he became Britain's first male Grand Slam singles winner since Fred Perry in 1936.

Four-time winner Federer, keen to extend his long stay at the top of men's tennis, opted out of playing a warm-up tournament. The Swiss has not reached the Melbourne final since his last win in 2010. But he said it had allowed him to feel fresh and eager for more Grand Slam glory as he prepared for his 53rd straight major.

On women’s side, Serena Williams is a red-hot favourite in her quest for a sixth title with even world number one Victoria Azarenka admitting the American is the player to beat. Williams gets her 13th Melbourne campaign underway Tuesday against Romania's Edina Gallovits-Hall, fresh from claiming her 47th career title in Brisbane and with a rare calendar-year Grand Slam in her sights.

"That's an incredible goal," she said of the Grand Slam of winning the Australian Open, Roland Garros, Wimbledon and the US Open in the same year, a feat last achieved by Steffi Graf in 1988. "It hasn't been done since the Eighties. I don't know if I can do it. Maybe someone else can. We'll see. But it's tough to say." Williams, who won Wimbledon, the US Open and Olympic gold last year, is also halfway to holding all the major titles at once for the second time, after first completing the non-calendar year "Serena Slam" in 2002-2003.

Azarenka, who withdrew from her Brisbane semi-final against Williams after failing to recover from a big toe infection, is happy for the American to be favourite, saying it takes the pressure off.  Reigning French Open champion Sharapova, who was beaten by Azarenka in last year's final, is one of the players wanting to topple Azarenka but her preparations have been hampered by a collarbone injury.

However, the world number two said she could play her way into the tournament, starting against fellow blonde Russian Olga Puchkova, ranked 105. "Yes, I might be a little bit rusty, but I'll work my way through it. I'm experienced enough to know the adjustments I have to make in those types of circumstances," she said. Former world number one Caroline Wozniacki could also be in contention, along with the in-form Agnieszka Radwanska, who has already won twice this year, and China's Li Na, the runner-up in 2011.