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.
And Britain's Neal Skupski and Dutch partner Wesley Koolhof - who are playing in their final tournament together - began their campaign with a 6-3 6-4 win over Australians Jason Kubler and Rinky Hijikata.
Alcaraz struggles for consistency
World number two Alcaraz missed last year's tournament with an abdominal injury and was making his debut at the season-ending men's event for the world's top eight players.
He had been hampered by foot and back problems last month and lost in his opening match at the Paris Masters on his return from injury.
His shaky form continued on Monday as he struggled for consistency on the fast indoor hard court against Zverev.
He trailed 3-1 in the opener before breaking back and staved off four break points in his next service game to lead the German 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.
But there was a worrying moment for the German - who missed last year's event while recovering from a nasty right ankle injury sustained at the 2022 French Open - when he fell on his left ankle towards the end of the match.
"I didn't twist my ankle. I kind of slipped. There was a pain for a while and I don't think any damage has been done," the ATP Finals website quoted Zverev as saying. "I hope not and we will see when it settles down. It is nothing comparable to Paris."
Medvedev halts Turin losing run
World number three Daniil Medvedev was defeated in all three of his round-robin matches in Turin last year, losing all of them in third-set tie-breaks.
But the 2020 champion has got off to a much better start this time and was in dominant form against compatriot Rublev, whose increasing frustration boiled over at times.
In a high-quality and draining first set played from the baseline, Medvedev broke in the seventh game but was then tested when serving for the set three games later.
The 2021 US Open champion had to save four break points - helped by some brilliant reflexes at the net - and squandered three set points with some loose play before finally converting his fourth.
Medvedev then ran away with the second, going a double break up after his opponent and good friend slipped on the court while reaching for a shot.
That left Rublev nursing his right hand after he landed on it and he sat with his head in his hands at the changeover, talking to himself and then taking his emotions out on a water bottle.
Medvedev quickly wrapped up victory, serving out to love, to go top of the group above Zverev because he won in straight sets.
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 Denmark's Holger Rune on Sunday.
The Serb is back in action on Tuesday when he takes on home hope Jannik Sinner (20:00 GMT), while Rune faces Greece's Stefanos Tsitsipas in the other singles match (13:30) in Green Group.
