“He was playing a solid match but I was making too many errors and doing too many bad things and that’s why I lost.”
Goffin, 26, is playing in his first end of season championships and he advances to the semis on the back of his triumph over Thiem and an opening match victory over top seed Rafael Nadal.
He will play second seed Roger Federer on Saturday for a place in Sunday’s final.
"I've never found a key to beat Roger," said Goffin. "Honestly, I don't know what to do. But I'm going to try something, something different.”
The 36-year-old Swiss, who is seeking a seventh end of season championships, has won all six of his clashes with the Belgian. The last encounter came in the semi-final at the Swiss Indoors in Basel in October where Goffin won only three games.
"I will try to do my best to play a better match than in Basel, for sure," he said. "In Basel it was not easy. He played well. He didn't miss. He was really aggressive. He returned so well.”
While Goffin lost his second match in the pool to Grigor Dimitrov, Federer moved into the semi-finals unbeaten in the Boris Becker group. It was the 10th time in 15 appearances that he had completed a clean sweep.
“Roger’s best surface is indoors,” said Goffin. “There is no wind. It's tough to get the ball to bounce high when you play him here on this kind of surface.It's perfect for Roger. If he's aggressive and he hits the ball really hard with his forehand, he's dangerous."
Tournament debutants Grigor Dimitrov and Jack Sock will contest the other semi-final.
Eighth seed Sock has won three of the four meetings with the 26-year-old Bulgarian. But the American, who saved four match points, before beating Dimitrov in the last 32 at the Indian Wells Masters in March, said he would be wary.
“I think Grigor’ has found his game and knows what he wants to do when he is on a tennis court.
He is playing deep into tournaments every week and has really established himself in the top 10.”
Sock added: “But I’m playing with confidence. I’ve won some matches to get into the semis. I’m proud of myself and looking forward to getting out there and competing.”