Rafael Nadal's latest bid to finally land the first ATP Finals title of his career started with a defeat by American eighth seed Taylor Fritz.
Fritz is making his debut at the season-ending event and his relentless returning was too much for Nadal.
In the same group, Norwegian third seed Casper Ruud made a winning start with a 7-6 (7-4) 6-4 victory over Canada's seventh seed Felix Auger-Aliassime.
The Turin tournament sees eight of the best men's players of the season split into two groups of four, with the top two qualifying for the semi-finals.
Spain's world number one Carlos Alcaraz withdrew because of injury, which led to first alternate Fritz taking his place.