"After the first match against Rafael I wasted so much energy, I was not ready for Grigor, but I had a good rest yesterday and was ready for today, and I hope I can do the same for the semi-finals".
But the world No.8 raised his game to deny the Swiss superstar a record 10th appearance in the title match - and the chance to crown his spectacular comeback year with a seventh World Tour Finals triumph.
Federer was quickly into his stride in front of a packed house at London's O2 Arena, winning the first set easily and the chances of a comeback from the Belgian appeared remote.
A bruising first set went in Sock's favour before Zverev took the second easily. It paid off. I started to serve well.
"So maybe the satisfaction is when I do win and I do show up at the best tournaments in the world, like here, I know it's not normal maybe to be here, so I appreciate it actually maybe even more so now at this age". They play later Saturday. I will watch that one tomorrow and just try to relax.
Having won all three of his round-robin matches at the O2 Arena, Federer continued his fine form to break twice in the first set.
From 0-3, 0-15 down in the opening set, the No. 7-ranked Goffin won 15 consecutive points to take control of the match, and never looked back.
And a frustrating match for the world number four came to a fitting end as Goffin wrapped things up by giving a poor second serve the appropriate treatment with a return to which Thiem could not respond to clinch second spot in Group Pete Sampras.
"In the beginning it was like being a kid in a candy store, sharing the locker room with legends of the game, seeing them prepare, being able to practise with them, playing doubles with or against them", Federer said, as quoted by the Express.
The winners flowed from Goffin's racket, unsettling Federer, who coughed up uncharacteristic errors.
Federer, 36, appeared more focused at the start of the third set but Goffin broke in the third game to take a 2-1 lead and heap the pressure on the Swiss.
The match was typical of Goffin's inconsistent week on his full debut at the tournament, after he played one match as an alternate a year ago.