Fed not moving as well – Djokovic

The pair face each other in their opening group clash at the ATP World Tour Finals in London this evening in a rematch of last year's final, which was won by Djokovic.

It has been a tough 12 months for Federer, who has won only one minor tournament this season, seen his run of consecutive grand slam quarter-finals ended at 36 and only just qualified for the season-ending event at the O2 Arena.

Back problems have undoubtedly been a factor but, while pundits have spent hours analysing what has gone wrong for Federer, his rivals have been reluctant to do the same.

Djokovic was happy to voice his thoughts, though, saying: "From my point of view he hasn't been moving as well as he did before. I guess that's one of the reasons why he hasn't had much success this year but he's Roger Federer, he 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."

A sign of Federer's struggles is that he and Djokovic will clash for only the second time this season. The first of those came on Saturday at the Paris Masters, when Djokovic fought back from a set and a break down to win in three sets.

The world number two then beat David Ferrer in the final before dashing to London on Sunday night.