Sock was bidding to become the first American ATP Finals finalist since James Blake in 2006. Federer - making his 15th appearance in the season-ending tournament - was riding on the back of a magical season.
The last player to win the trophy in their first year as a qualifier was Alex Corretja in 1998. One man's genius against another's grit.
"I have no words I can not describe how I'm feeling, so much joy, so much happiness". To beat Roger here in front of all the crowd, is such a great feeling.
"Things went my way tonight and I'm more than happy".
"One person that, I don't know if she's in the hall or not, but she played a pretty important part, was my girlfriend Nicole".
Grigor Dimitrov had already qualified for Saturday's ATP Finals semifinal when he stepped out onto court on Friday, but that didn't prevent him from blowing Pablo Carreno Busta off the court with a crushing 6-1, 6-1 victory.
Down 0-3, Jack saved a crucial break point hitting a fast forehand, then he started to be more consistent and solid and broke back his opponent. Goffin was certainly displaying a more attacking performance than in his previous matches - pelting balls into either corner and mixing the game up with sliced drop shots. Who will win the World Tour Finals this year? At 3-0 in the second set, Goffin was digging his heels into the game.
To Goffin's credit, he stuck with the task, running down everything, and consistently asking the tough question. In the second game of the first set Sock made two forehand errors and a double fault, Dimitrov was smart enough to play an aggressive tennis and break Sock at 30.
A surprisingly flat Federer could not muster another challenge as Goffin served out in style to reach the biggest final of his career and guarantee a new champion on today night. By then Federer, in ink-green shorts and a colourless tee, had replaced his match face, calculated calm, with a scowl, and his game had tightened too.
On Saturday, it appeared he would have an extra day to prepare for national duty as 36-year-old Federer feasted on the elegant Goffin's medium-pace game to boss the opener. He took this one. But Goffin was up for the ask, finishing with a wide swinging serve that brooked no answer from the champion Swiss. Everything seemed to be going wrong for the seventh seed, throwing away yet another break as Federer slowly found his groove.
Goffin took the second set with a single break of serve and had chances at the start of the third set.
"It was a tough game [at 2-3] and I was a little bit exhausted in the third".
"It's disappointing to finish on this note".