Carlos Alcaraz suffered defeat on his ATP Finals debut as Alexander Zverev fought back to win in Turin, Italy.
Wimbledon champion Alcaraz lost 6-7 (3-7) 6-3 6-4 to Germany's Zverev, who won the event in 2018 and 2021.
It is the first time the 20-year-old Spaniard has lost three matches in a row since March 2021, when he was ranked outside the world's top 100.
In the day's other Red Group singles match, third seed Daniil Medvedev beat fellow Russian Andrey Rublev 6-4 6-2.
In the doubles, Briton Joe Salisbury and American partner Rajeev Ram made a winning start to their title defence with a 6-3 6-4 victory over Australian Matthew Ebden and Rohan Bopanna of India.