Goffin, who beat an injured Rafael Nadal in the opening match but lost to Thiem in the second, needed only 72 minutes to win 6-4, 6-1, setting up a showdown with six-time champion Roger Federer on Saturday for a place in the title match.
Federer, who has won the ATP Finals six times - in Houston, Shanghai and London - has enjoyed a remarkable 2017 season after a long layoff in 2016.
Goffin, thrashed midweek 6-0, 6-2 by Dimitrov, served it out nervelessly as the Swiss seemed to slow up, and failed to recover from an early break of serve in the decider.
David Goffin got a chance to experience the ATP Finals action past year when he stepped in after Gael Monfils was forced to withdraw, and 12 months later he came as a regular competitor of the elite ATP event, occupying the 8th place in the ATP rankings. It is the first time in the tournament's history that three debutants have reached the semis.
"Both are really special", the Belgian said, when asked about the significance of the victory in his post-match press conference.
"It is the greatest win of my career, for sure".
Dimitrov and Sock will play later on Saturday for the right to meet Goffin in Sunday's final.
But Goffin is made of sterner stuff. Federer wasted two game points on his serve before Goffin produced a forehand passing shot to take his first break point of the match.
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.
Indeed, this had all the makings of a mismatch from the moment Federer converted a fifth break point in the first game. Consecutive aces gave him a 30-0 cushion before he put a nervy forehand in the net.
"At the end I was happy with the way I managed the match".
"It's disappointing to finish on this note".
"I'm feeling okay. It was not the problem today". "But I had my chances and missed them", he said.
Sock channelled that rage with a burst of power hitting as he won four successive games.
However, the match turned in the second game of the second set.
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