Novak Djokovic pinpointed a decline in Roger Federer's movement around the court as a major factor in his struggles this season.
The pair face each other in their opening group clash at the Barclays ATP World Tour Finals in London on Tuesday 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 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.
Facing Federer first up would not have been his ideal scenario, and Djokovic said: "It's right away probably the biggest challenge that I will have.
"I have to recover and get ready for these conditions and try to play as well as I did in (Paris) Bercy.
"If I do that then I think I have a good chance. But of course it's the first match for both of us and I did come a day later so we'll see how that will work to his advantage or my advantage."
The first match of the day is a rematch of the other Paris semi-final between Ferrer and Rafael Nadal.
World number one Nadal had won nine straight matches against his fellow Spaniard prior to Saturday, and he will hope the slightly slower conditions in London play to his advantage.
Because there was no gap between the two tournaments - there will be a week again next year - organisers were forced to split the groups for the first two days of play at the O2.
In Group A on Monday, debutant Stanislas Wawrinka upset Tomas Berdych 6-3 6-7 (0/7) 6-3 while in the Group B opener Juan Martin del Potro came from a set down to defeat Richard Gasquet 6-7 (4/7) 6-3 7-5.