"I think I let that get me down a lot through the year", Sock said.
A bruising first set went in Sock's favour before Zverev took the second easily.
The second set proved to be a one-sided affair as Goffin dominated, dropping just one game on his way to securing a place in the semi-finals.
Sock has been a breath of fresh air, treating the crowds to trick shots as well as his cannonball forehand and his semi-final against fellow first-time qualifier Grigor Dimitrov on Saturday promises to be a feast of spectacular tennis.
The World No.2 looked too uncharacteristically agitated on court to suggest he was competing for a flawless Round Robin performance, but it seems the heightened expectations were taking their toll. Somehow after the three-love [game], I lost it until the end.' He played 27 tournaments this year.
"I never realised! This is how insane we are these days".
"Of course, the US Open was very painful".
By now, Roger looked to have relaxed into the game and closed in on a quick deciding set in just 26 minutes.
The first set went with serve all the way through to 6-6, but the seven-time champion never really looked to have turned on the gas; throwing away easy points with careless backhands into the net and then dropping in a few casual aces to keep the scoreline ticking along.
Despite frequent debate over the last 12 months regarding when Federer's playing days will come to an end, the Swiss star has had one of his best seasons to date.
His 6-0 6-2 thrashing of Belgium's Goffin in the O2 Arena followed Monday's tense three-set win over Dominic Thiem when he became the first Bulgarian to take part in the elite tournament for the top eight players in its 48-year history.
"I'm extremely happy. It's been a tough group so to be there in two matches is great".
Steve Kerr on Kyrie Irving with Celtics: 'It's his team'
Rookie forward Jayson Tatum has also been a revelation and is among the leading candidates for Rookie of the Year so far. They obviously valued Irving as a rare superstar worth chasing. "But they sure look like they want it to be right now".