Novak Djokovic believes that Roger Federer's slump in form in 2013 is down to a lack of mobility in his game but insists that the Swiss is still a "class act".
The two are preparing to meet for their opening match at the ATP World Tour Finals on Tuesday at the O2 in a repeat of last year's final, which was won by the Serbian, denying Federer a third consecutive victory at the tournament.
"From my point of view he hasn't been moving as well as he did before," Djokovic told the media. "I guess that's one of the reasons why he hasn't had much success this year.
"But he's still Roger Federer, a 17-time Slam winner, who has achieved so much in his career and he's never to be underestimated as long as he plays tennis.
"He has an incredible quality in his game and he's still striking the ball really well. If he feels well on any day, he can beat anybody."
Djokovic beat Federer in the semi-finals of the Paris Masters last week.
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