There is a question for Thiem today, of how much he puts into the match against Rublev, especially if he loses a tight first set. He has a good reason to relax and conserve energy if that happens, but against this, he doesn't want to break a run of wins in the tournament. He's won the group and will play Zverev/Djoker in the semi, so I think he'd like to maintain his level. And of course, you don't want to give a player like Rublev a notch on his belt.
Tanking in tennis is a rare, low level event, usually committed by lowlifes like Tomic and Nick, but there are examples of the best players letting a set go to conserve energy for the next set, or even letting a match slide when they don't feel the gains outweigh the loss.
EDIT: he's rapidly gone 0-3 with two breaks of serve so I wonder if he's put his feet up already?