It was 4-3 Dimitrov when he decisively broke serve to put himself on the brink of the final.
"I've never found a key to beat Roger", Goffin said after defeating Thiem on Friday. It was just the feeling.
For Sock, it was hard to even predict he would be here at the O2, and yet he saw himself seeing off two-times Masters 1000 victor Alexander Zverev and Wimbledon runner-up Marin Cilic to join Federer in escaping from Group Boris Becker.
The Belgian opened the match well, acing Federer twice, but he threw away a game point allowing his opponent to swoop in and claim the first game easily.
The first game of this interesting encounter was already a classic, with Goffin having to fend off the merciless offense from Federer immediately from the first rally.
But 22 minutes in and the world No. 2 held a comfortable 4-1 lead. Alas, Federer countered with deep crosscourt angles and stepped in to punish when many of Goffin's balls fell short of the baseline. At 3-0 in the second set, Goffin was digging his heels into the game.
The American brought up two set points and took the first of them when Dimitrov dragged an attempted backhand return from a second serve into the net. Cheers of support for the underdog were soon muted by the hollers of "come on Roger!" at the beginning of the third set.
The second game of the second set was to prove a turning point as Goffin, beaten 6-2 6-1 by Federer in Basel last month, broke the Swiss's serve with a forehand victor and then consolidated his lead to take the match into a decider.
This was certainly a different Goffin; forcing Federer to counterattack with attack.
Dimitrov wins ATP World Tour Finals title
Dimitrov needed to answer and did so to hold serve and go 4-3 in front. A hold from Goffin and a hold from Federer and the Belgian was serving for the match.
"I have no words. But in the end, I was really happy about the way I played the match, I was really nervous towards the end of the match", Goffin said in his on court interview after the match. But like Sock, he's also a first-time Masters 1000 victor this year, in Cincinnati.
He was the better player early on, using his pinpoint accuracy to keep Dimitrov under control. "Somehow after the 3-0, I lost it until the end".
"To beat Roger for the first time here in such a big event, big tournament, it was the ideal moment", said the victor.
Sock and Dimitrov have been involved in some real humdingers in recent times, with the American saving match point in each of their last two meetings and going on to win. It was turning into quite the contest.
And he continued his surge in the first to break for 5-4 and see out the set in style.
Sock won the ATP Finals shootout with Alexander Zverev, beating the German in three sets to secure a semi-final berth in their last group match.
However, in the major moments Goffin was able to keep his game under control, taking five of his seven break point opportunities. And what an incredible set of tennis the fans were gifted with as a result; drops shots, volleys and ace aplenty.
The pair were well beaten 7-6 6-2, with Kontinen/Peers set to meet Lukasz Kubot and Marcelo Melo in the final, after their win over Ryan Harrison and Michael Venus. I just want to keep the momentum going. Arguably, Goffin played his most clutch tennis in the tournament after he managed to fend off the fightback from Federer, holding onto his lead narrowly with some tight service holds. Knows his way around the court very well.