Austria’s Thiem, who beat Tsitsipas and Nadal earlier within the week to win Group London 2020, was nicely under his finest at London’s empty O2 Arena in a 6-2, 7-5 defeat.
Rublev, 23, broke the third seed twice to race right into a 3-0 lead at first of the spherical-robin match.
Thiem stopped the rot and began holding his serve however couldn’t make inroads into the Russian’s serve and the set was over in simply 25 minutes.
US Open champion Thiem, enjoying considerably under par, tried to dig deep in opposition to Rublev, who has had a breakthrough season, successful 5 titles.
But the Russian, who was within the groove, handed Thiem within the third sport to interrupt for the third time within the match and set up an iron grip.
Just when it appeared seemingly he would canter to victory his degree dipped and he dumped a forehand into the web at hand Thiem a break, smashing the ball away in frustration.
At 4-4 the Austrian gave the impression to be again within the contest however was damaged once more within the eleventh sport, sending an angled backhand huge.
Rublev missed a straightforward overhead as he served for the match however sealed the win with an ace.
“I started really well since the first point, the first return and I think Dominic didn’t start that well,” stated Rublev.
“I took advantage from the beginning and that helped me to have more confidence and helped me to play better and better.
“It was not straightforward for Dominic however I believe he’s additionally centered now already on the semis so I want him the very best of luck. He deserves to be the place he’s and he deserves to win the title.”
French Open champion Nadal, in search of to win the ATP Finals for the primary time in his illustrious profession, has a 5-1 successful file in opposition to Tsitsipas however the Greek will take confidence from final yr’s win in London.
Daniil Medvedev has already certified from Group Tokyo 1970 — 2018 champion Alexander Zverev and high seed Novak Djokovic will battle it out on Friday to affix him within the semis.
The O2 Arena is internet hosting the elite eight-man occasion for the twelfth and ultimate season earlier than the championships transfer to Turin.