Novak Djokovic claims Roger Federer's decline is partly down to his "slower" movement around the court.
The pair face each other in their opening group match at the Barclays ATP World Tour Finals in London on Tuesday night, a year after Djokovic beat Federer in the final.
Federer has won only one minor tournament this season and saw his run of consecutive grand slam quarter-finals end at 36.
Back problems have undoubtedly been a factor and although pundits have spent hours analysing what has gone wrong for the Swiss, his rivals have been reluctant to do the same.
Djokovic was happy to voice his thoughts on Monday, 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.