"We need some days off to rest and I will be ready next weekend", said Goffin.
"It is such an honour to play here. This week has been one of the best I've ever had".
Thiem, though - also on a recent run of bad form - was delivering his share of mindless errors; one particularly mistimed backhand fired off the frame and well up into the stands, and was met with a pantomime "ooh" from the crowds.
"It was an unbelievable effort".
Growing in confidence, the 26-year-old broke Federer's serve again at 1-1 in the third set as the Swiss, suddenly hesitant, struck a backhand out.
Belgium's David Goffin shakes hands with Switzerland's Roger Federer after winning their semi final match.
Goffin missed Wimbledon after injuring his ankle at the French Open but is enjoying a dream end to 2017 with next week's Davis Cup final in France still to come.
"It was a special week for me", he said. It was a week with a lot of emotions, a lot of fatigue of course.More news: Rose McGowan says she will plead not guilty to drug possession
"I have no words I can not describe how I'm feeling, so much joy, so much happiness".
The win will propel Dimitrov to a career-best position in the ATP rankings, where he will finish the year as third in the world, irrespective of the outcome of the final against Goffin.
Goffin, who became just the sixth player in history to beat Roger Federer and Rafael Nadal in the same tournament, forced his opponent back in the opening game and broke in stunning fashion with a scintillating forehand victor. 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.
But Goffin broke serve with a forehand victor in the second game of the second set and consolidated his advantage to take the match into a decider.
Serving to stay in the first set at 6-5, Goffin saved four set points and then had the chance to close out the game but instead double-faulted.
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.
Goffin was left serving for survival at 5-3 down and clawed his way back from 40-0 down to stay alive in an enthralling encounter, but a exhausted backhand into the net gifted Dimitrov the title in the next game.
He is the first man to beat both players at the same tournament without taking home the trophy but put that disappointment aside to say he has improved as a result of his eyecatching week.