A former tennis star has hailed Roger Federer's use of a brilliant serving technique that "defies logic" and differs significantly from great rival Rafael Nadal.
Former British number one-turned tennis pundit Greg Rusedski was in awe of the Swiss maestro's come-from-behind win over Marin Cilic during his post-match analysis for Sky Sports.
Federer notched yet another milestone at the ATP World Tour Finals in London with a 6-7 (5-7) 6-4 6-1 victory on Thursday to reach the last four unbeaten for an incredible 10th time.
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Already assured of a record 14th semi-final appearance at the season-ending showpiece, Federer clinched top spot in his four-man group courtesy of the win.
After the victory, Federer was asked to analyse his serve compared that that of world number one Nadal.
Footage showed that when both players tossed the ball in the air, Federer took a glance at his opponent before serving, while Nadal's eyes remained firmly fixed on the ball.
"I think it is something that just happens," Federer said.
Federer catching a glimpse at his opponent mid-serve. Pic: Getty
"I think Marin is also similar to Rafa, he goes up first then the ball toss comes.
"I'm not sure why I do it. Maybe I feel like it gives me one last [piece] of information. But at the same time it can throw you off if you see him shuffle around, I don't know.
"But this is something from junior days, you just bring it over to the pro game I believe."
While the Swiss great's self-appraisal was fairly nonchalant, Rusedski - a man renowned for his serving prowess as a player - explained how uncommon Federer's technique actually was.
"That is quite unique with Roger being able to do that, look at his opponent mid-toss," Rusedski said.
"That's something that's natural (to him).
"He defies logic what he can do out there.
Nadal focuses intensely on the ball when serving. Pic: Getty
"And that’s another thing that Federer does that’s very unique."
Federer will face the winner of Saturday's clash between Dominic Thiem and David Goffin for a place in Monday's championship decider.
The 19-time grand slam champion admitted it was a strange feeling knowing he wouldn't be going home even if he had lost to Cilic, but said the unusual scenario also freed him up to play his best tennis when he needed it.
"I tried to relax a little bit," he said after completing the round-robin play with three wins from three.
"But of course I want to do well. I want to try to keep the momentum going and play the right way and, to do that, a win's always very helpful."