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.

Serbia's Novak Djokovic, aiming for a record-equalling sixth title, starts against Greece's Stefanos Tsitsipas on Monday, with Daniil Medvedev facing his fellow Russian Andrey Rublev in the other contest.

While there are no British players in the singles event, three men from the nation are competing in the doubles.

Former world number one Joe Salisbury, seeded second with American Rajeev Ram, won 6-3 6-7 (6-8) 10-8 in a rollercoaster match against seventh seeds Marcel Granollers and Horacio Zeballos on the opening day.

Lloyd Glasspool and his Finnish partner Harri Heliovaara are making their debut in the event and the sixth seeds opened with a 7-5 7-6 (7-3) triumph against Marcelo Arevalo and Jean-Julien Rojer.

Neal Skupski, who has been part of the world's best team this year, plays on Monday.

Skupski and Dutch partner Wes Koolhof have enjoyed incredible success in their first year together, winning seven titles which have included Masters 1000 victories in Madrid, Montreal and Paris, and finishing 2022 as year-end world number ones.

The pair open Monday's play in Turin against Australians Nick Kyrgios and Thanasi Kokkinakis.