Defending champion Novak Djokovic will be bidding for his third consecutive Grand Slam trophy when the Australian Open champion gets his title defence under way in Melbourne.

The Serbian World No. 1 dominated last season, winning three of the four Grand Slams and goes into the year's opening major as the man to beat against chief rivals Rafael Nadal, Roger Federer and Andy Murray.

Djokovic kicked off his sensational 2011 with victory over Murray in the Australian Open final and then added the Wimbledon and U.S. Open crowns, beating Nadal both times in the title matches.

Only four men on five occasions have won three or more consecutive Grand Slams in the open era — Rod Laver, Pete Sampras, Nadal and Federer twice.

The Australian Open, starting on Monday, is the toughest of the four Grand Slams to hold onto with only nine successful title defences in the open era compared with 17 at Wimbledon and 13 at Roland Garros.

Last year's Australian Open final triumph over Murray was the first of seven straight tournament wins in a 41-match winning streak that was finally ended by Federer in the semifinals of the French Open.

Djokovic, 24, won a career-best 10 titles in 11 finals last year. He defeated World No. 2 Nadal six times and overall was 21-4 against top-10 opponents.

But former Australian Open champion Nadal said he was over the shoulder injury that dogged him late last year and was determined to challenge for the top ranking he lost to the Serb at Wimbledon in July.

Chequered record

Ten-time Grand Slam champion Nadal, 25, has a chequered record at the Australian Open, winning in 2009 but failing to make the semifinals in three of the past five visits.

Federer, who holds the all-time record for Grand Slam wins, is bidding to become only the second man along with Roy Emerson to win five or more Australian Opens.

World No. 3 Federer, who won his 16th Grand Slam title in Melbourne in 2010, could also end up overtaking Jimmy Connors's record of most open era grand slam matches (233) if he reaches this year's final.

The Swiss great, competing in his 49th consecutive major, is the oldest of the men's top four at 30 but his durability always makes him dangerous in the second week of a major tournament.

Sense of uncertainty

Meanwhile, the women's draw is one of the most open in years and adding to the sense of uncertainty in Melbourne are recent injuries to superstars Kim Clijsters and Serena Williams, who share 17 major titles but are both outside the top 10.

Belgium's Clijsters, the 11th seed, is defending champion after beating Li Na in the 2011 Melbourne final, before the Chinese player went one step further at the French Open.

Second seed Petra Kvitova of the Czech Republic won on grass at Wimbledon and Australia's Samantha Stosur took the U.S. Open title, upsetting home favourite Serena in the September 11 final in New York.

The name missing from the 2011 list of winners is top seed Caroline Wozniacki, yet to win her first Grand Slam, whose World No. 1 ranking is under imminent threat from rising star Kvitova.

The two rivals have already clashed this year, with Kvitova of the Czech Republic winning in three sets at the mixed-teams Hopman Cup in Perth, which does not carry ranking points.

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