"I never thought I'd be getting this trophy this time a year ago", the Spaniard said.
"This is the best I've felt since the (Juan Martin) del Potro final (in Basel)".
More stoic resistance by Sock saw him fend off more danger at 5-5 but Federer remained untouchable on serve and won 29 of 32 points on his delivery in the second set.
Murray and Djokovic were joined on the sidelines for the majority of the season by Kei Nishikori, Milos Raonic and three-time Grand Slam champion Stan Wawrinka - all of whom ended 2016 in the world's top-five.
Sock started the second set in style, holding to love in a game sealed by a stunning cross-court forehand that left Federer stranded.
Federer, the oldest player to qualify for the ATP Finals since 1970, looked aghast as two break points went begging at 3-3 and another two at 4-4 in the second set.
"When he hit it, it wasn't very clean. It landed just inside both lines", said Sock, who still won the game. It's Roger, I guess.
Another light-hearted moment came when Sock hit a shot that should have set up an easy victor for Federer.
"It was a fearless move for him to just put himself out there, give it a go", said Federer.
Federer, surprisingly, put his easy chance into the net. "The crowd, the atmosphere is wonderful", Zverev, who reveled on the big stage, told reporters after the late-night finish.
Federer has already picked up seven titles this year which included the Australian Open, Wimbledon and will look forward to chase his first season-ending title since 2011. "That's what I should have aimed for", he joked. 2 emphasised that the long format did not require much tampering as it helped a player to stretch a lead.
After the Federer match, Rafael Nadal was given his trophy for finishing the year as the No. 1 player in the rankings. World number one Nadal, who won the French Open and US Open this year, admitted on Friday that he is not training at 100 percent ahead of his tournament opener against Belgium's David Goffin on Sunday, although he said he is likely to be fit enough to play.