Number one seed, Novak Djokovic, eased to a straight-set – 6-3 7-6 (7-4) – win over Alexander Zverev to clinch a place in the last four of the tournament at the O2 Arena.
Zverev brought more fight in the second set but Djokovic raised the bar when it mattered in the tie-break.
The 33-year-old will be up against US Open champion Dominic Them in Saturday’s semi-final.
Thiem progresses as the winner of Group London 2020 while Djokovic goes through Group Tokyo 1970 as runner-up.
“I felt great. In the first set, he had a couple of break-point chances and I managed to serve well in the important moments,” said Djokovic.
“In contrast to the match against Daniil, I found the right shots at the right time. I have tremendous respect for Sascha, who is a great player and it is not easy to return 140mph first serves.
“It was anybody’s game for most of the match.”
Russian fourth seed, Daniil Medvedev, was already assured of topping the group after beating Djokovic on Wednesday, but there was no drop in his level as he beat Argentina’s Diego Schwartzman 6-3 6-3 in their dead rubber.
In the day’s other game, Daniil Medvedev, having already secured his place on top of the group after beating Djokovic on Wednesday, beat Argentine Diego Schwartzman in straight sets (6-3 6-3) in a dead rubber affair.
Medvedev will now face Spanish second seed and 20-time Grand Slam champion Rafael Nadal – who is targeting his first-ever ATP Finals trophy – in the semi-final on Saturday.