Rafael Nadal's bid to end the year as world number one faltered with a straight-set loss to defending champion Alexander Zverev at the ATP Finals.
The Spaniard, a fitness doubt before the season-ending event in London with an abdominal injury, was beaten 6-2 6-4 in their opening round-robin match.
Nadal, who has not completed a tournament since September's US Open, was out of sorts and error prone.
Earlier, sixth seed Stefanos Tsitsipas beat fellow debutant Daniil Medvedev.
The Greek 21-year-old beat the Russian 7-6 (7-5) 6-4 at the O2 Arena.
Novak Djokovic, who can overtake Nadal at the top of the world rankings this week, leads the other group after winning on Sunday.
The top two players in each group after the round-robin stage progress to the semi-finals on Saturday.
The men's season-ending event, which is taking place in London for the penultimate year, features the top eight players of the year.