Nishikori shocked Federer with a 7-6(4), 6-3 victory in his opening ATP Tour Finals match last night.
But the Japanese star could not have envisaged such a result 12 months ago when he was sidelined with a wrist injury.
When coming back from the problem at the start of this season, Nishikori dropped down to play on the Challenger Tour as part of his rehabilitation.
He won the RBC Tennis Championships of Dallas and was quickly back competing with the best again, losing to Rafael Nadal in the Monte Carlo Masters final in April.
Not many top level players would have taken the decision to drop and play at less prestigious events, but Federer hailed Nishikori’s bravery.
“I’m really happy for him that he qualified for this event,” Federer said.
“Not taking me out today – but I’m happy for him because he started playing challengers.
“That was a bold decision early on. Yeah, that deserves credit and respect.
“I think we all have that for Kei because he’s a great fighter.
“I thought we played a great match against each other in Shanghai. I think in Paris, maybe also a little bit tired from Vienna the previous week.”
Federer will take on Dominic Thiem in his second round robin match of the ATP Finals after the Austrian lost to Kevin Anderson.
But he has not yet started to think about that tie.
“We haven’t played in a while, I don’t remember when that was the last time. I mean, okay, whatever,” he said.
“I haven’t thought about the match, to be honest. I haven’t had enough time. I wasn’t even thinking who I’m going to play next.
“I just know I need to do better than today.
“That’s pretty much it.”