Federer has enjoyed many remarkable days during his illustrious career, but even the 16-time Grand Slam champion was surprised by how completely he overwhelmed his old rival at London's O2 Arena on Tuesday.
The 30-year-old took just an hour to crush Nadal with the kind of imperious display that ranks alongside the very best Federer has ever produced.
Even when Federer had beaten Nadal in the past he has usually been pushed to the limits in the process.
Nadal may have been slightly affected by the illness and injury that has dogged him in recent weeks, but Federer had never beaten the Spaniard by such an emphatic margin before and the dominant manner of the victory made it one of his all-time great performances.
"It's definitely one of the nice ones, that's clear. I had some good ones in my lifetime but this one ranks high because it's against my biggest rival," Federer said.
"It's the World Tour Finals, where it really matters. That I've been able to come through so convincing is a bit of a surprise to me, but it ranks extremely high.
"Possibly he struggled more early on indoors in only the second match back from five weeks out.
"But to come through so convincingly is great for me because we know Rafa has a certain standard which he does not go below.
"That's where this victory, still for me, I can rate this extremely high."
Federer's struggles against Nadal have been well documented and he went into the Group B encounter on the back of three successive defeats against the Spaniard.
But 12 of Nadal's 17 wins against Federer have come on his favoured clay and the Swiss star has always fancied his chances more on the quicker hard courts.
Federer -- the first player to book his place in this year's semi-finals -- never gave Nadal time to get into a rhythm as his cleverly varied serves and sublime ground-strokes set a daunting tempo that the world number two couldn't match.
"Of course, it was a great match for me basically from start to finish," Federer said.
"I was able to do what I was hoping to do; dominate from the baseline, play close to the baseline, serve well, take his time away.
"I was able to do that tonight when it hasn't always worked. I've also felt the power of Rafa in the past, so this is a great match for me."
Although Federer will finish the year without a Grand Slam title for the first time since 2002 and has slipped to fourth in the rankings, he is determined to prove wrong the critics who have written him off as a player in decline.
And he insists that Nadal never had any kind of mental hold over him despite the Spaniard's head to head dominance.
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"I always knew I could beat Rafa. The question is sometimes it was hard to do because he has a big say, as well, in how the matches go. The quicker the court, the more I favour myself," Federer said.
"I'm happy I was able to use the conditions to my advantage. Maybe Rafa didn't play his very best.
"We've played so many times, sometimes it just derails for you, like it derailed for Rafa today.
"I don't know how much it is mental. Once you're down a set and a double break, you're not even playing to win anymore, you're just hoping to stay in the match.
"It's not the finals, I know that, but it's still a stepping stone to the finals.
"I'm one match away from that now. I'm very happy to have qualified, because also that pressure is gone."