Carlos Alcaraz beaten by Alexander Zverev on ATP Finals debut.
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.
Briton Joe Salisbury and American partner Rajeev Ram are also in action later on Monday against Australia's Matthew Ebden and Rohan Bopanna of India.
Alcaraz had won his past two meetings with Zverev this year but struggled for consistency on the indoor hard court. He trailed 3-1 in the opener before breaking back and staved off four break points in his next service game to lead Zverev for the first time.
Zverev himself saved two set points to force a tie-break, but Alcaraz found some semblance of rhythm to clinch the opener.
However, seventh seed Zverev responded instantly, breaking Alcaraz at the first opportunity as he forced a deciding set. Zverev played the big points better throughout, saving five of six break points and hitting 16 aces before sealing victory with a service winner.
"It's nice to beat the best players in the world, and he's definitely one of them," Zverev said.
"It's special to win on this court again."
Russia's Daniil Medvedev plays compatriot Andrey Rublev later in the Red Group.
The ATP Finals features eight of the year's best men's players split into two groups of four, with the top two qualifying for the semi-finals.
Novak Djokovic is the defending champion and secured the year-end number one status with victory over Holger Rune on Sunday.