Tennis: Djokovic challenges Federer to 'keep up'

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World number one Novak Djokovic threw down the gauntlet to a resurgent Roger Federer on Saturday by challenging him to 'keep up' as the Serb bids to repeat his landmark 2011.

Djokovic, who named the London Olympics as a top priority for the year, shrugged off the all-time grand slam champion's strong finish to last season culminating in victory at the ATP World Tour Finals.

The 24-year-old Djokovic went 70-6 last year and won 10 of 11 finals including three grand slams, starting with his demolition of Andy Murray at the Australian Open in January.

And Djokovic, speaking before beginning his title defence in Melbourne, warned he was primed to repeat his golden season, despite Federer's late-year hot streak.

"Well, he finished off the season best from all the other players. He had over 15 wins in a row. He definitely loves playing indoors. He loves playing in the London event," Djokovic said.

"But, you know, it's a whole new year. It's a whole new season. We're starting to play outdoors. We'll see if everybody can keep up."

Djokovic said he had taken extra rest in the off-season to shake off the shoulder and back problems that overshadowed the final months of last season, but trained on Christmas Day during a low-key festive period.

And he said he was targeting this year as a re-run of 2011, when he strung together what has been described as the greatest campaign in tennis history.

"Everything is possible. Obviously 2011 has been the best year so far in my career. It's going to be very difficult to repeat what I have done," Djokovic told journalists before the season's first grand slam.

"But look, I've done it once. Why not twice? Why not staying optimistic and positive about the whole season? It's a start.

"Obviously I'm not thinking too far away from Australia. My focus is directed to this tournament. I want to start off the year well, as everybody else (does) obviously."

Meanwhile Federer said he had recovered from the back spasms that forced him to pull out of the Qatar Open semi-finals, interrupting a 20-match winning sequence.

"Today was my first practice where I could play again at 100 percent," said the Swiss.

"Yesterday I felt good, too. No pain. But at least, you know, I was out there playing full-on, but still just a little worried or scared, let's put it that way.

"Today all that's gone, so I feel like I'm back to normal. That's a good feeling to have coming into the Australian Open now."

The 16-time slam-winner, now 30, will enter his 49th consecutive grand slam in Melbourne as he aims to end a two-year major title drought -- his worst spell in a decade.

"I think it's only helpful that I finished so strong. I had so many great finishes to the year," added Federer, a four-time winner at Melbourne Park.

"I remember every time it has helped me to have a good mindset on vacation, during the build-up, then at the beginning of the year. Very often did I take this momentum into the following year."

World number two Rafael Nadal has also had fitness concerns in the build-up to the Australian Open, while fourth-ranked Andy Murray is still targeting his first grand slam win.