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.
World number two Alcaraz qualified for the tournament last year but withdrew with an abdominal injury.
Earlier, Britain's Neal Skupski and Dutch partner Wesley Koolhof began their campaign with a 6-3 6-4 win over Australians Jason Kubler and Rinky Hijikata.