Close to three million people have attended the tournament since 2009 but sadly that tally will not be added to and silence greeted Thiem's entertaining 7-6 (5) 4-6 6-3 success against reigning champion Tstisipas and Nadal's straightforward 6-3 6-4 win over Russian debutant Rublev.
Nadal, who beat Rublev in straight sets in their opener, missed a chance to qualify with a match to spare and next will play Tsitsipas on Thursday with a semifinal place at stake.
"Always the first match is very tricky", he said.
"Today was a little bit different", Thiem said comparing this year to last year.
In a repeat of last year's final, Thiem was able to take revenge and beat the defending champion.
The serve in particular was a big difference between the two, with Rublev not creating a single break point and becoming increasingly frustrated as the match wore on.
The Spaniard has won just one title on indoor hard courts, which came back in 2005. Every win against a Top 10 player at the Nitto ATP Finals is something special and every win against Stefanos is something special, because he is such a great player.
Prediction: Tsitsipas in three sets.
Tsitsipas responded well to win the second set but Thiem upped his level and intensity in the decider and served out a valuable victory that sets up an intriguing clash with Nadal on Tuesday.
"He dealt with my serve better and took time to step back and apply more pressure to me", Tsitsipas said.
'Against Rafa it's nice to win the first set obviously but still he's there 100 per cent from the first to the last point so I knew that maybe I have a slight advantage but still, I had to stay super focused.
Thiem mixed things up against Tsitsipas, and while he is a solid volleyer, he tends to come in to finish his points off at the net but threw in a few serve-volleys against the Greek. "I didn't suffer much and that helps to play more relaxed on the returns". "I was not serving good at all because I was a little bit nervous, which is normal", Rublev said, per the Nitto ATP Finals website.
Now Nadal has to do it the hard way if he is to land the biggest prize missing from his trophy cabinet after losing the 2010 and 2013 finals at the prestigious season-ending event.
With Nadal never at his best inside the O2 Arena, this is a good opportunity for Thiem to start 2-0 in London for a second straight year. "I think playing like this I am confident that I can have my chances in the tournament".