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.
Novak Djokovic, who can overtake Nadal at the top of the world rankings this week, leads the other group after winning on Sunday.