Novak Djokovic saw his bid for a sixth ATP Finals title suffer a setback on Wednesday night after the world No 1 was soundly beaten by Russia's Daniil Medvedev.
Djokovic could have booked his place in the semi-finals following his straight sets win over eighth seed Diego Schwartzman in his opening Group Tokyo 1970 match, but instead it was Medvedev who guaranteed his passage with a 6-3, 6-3 win over the top seed.
Earlier, German fifth seed Zverev beat Schwartzman 6-3, 4-6, 6-3, bouncing back from defeat to Medvedev in his opening match.
Top seed Djokovic is seeking to match the record of six-time winner Roger Federer, who is absent from the season-ending tournament with an injury.
But Medvedev, who won the Paris Masters last week, beat Djokovic at his own game, grinding him down in a near-flawless defensive display.
The Russian earned a clutch of early break points and eventually made one count in the seventh game, which lasted 11 gruelling minutes.
Medvedev, 24, held serve easily in the next game and seized the opening set when a rattled Djokovic double-faulted.
An increasingly confident Medvedev broke the 17-time Grand Slam champion again at the start of the second set to win his sixth game in a row in the round-robin match.
Djokovic stopped the slide on his own serve but was able to force just one break point, which he could not convert.
The victory takes Medvedev through to the semi-finals but five-time winner Djokovic still has a chance to progress in a shootout with Zverev on Friday.
"Always I like to pay Novak, first of all because he's one of the greatest champions in the history of our sport and when I was eight years old I was already watching him on TV winning Grand Slams," said Medvedev.
"He was still young – so it was always a dream come true to play against him."
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"I was serving good and playing safe enough in the most important moments and that's why I got the win," added the Russian, who lost all three of his matches on his debut last year.
Djokovic has until Friday to regroup and will take on Zverev in what will be a winner-takes-all contest. The 34-year-old Serb has played the German twice at the ATP Finals, with Zverev winning the first encounter in the 2018 final, before Djokovic took his revenge 12 months later in a group stage match.
“He was just better, no question about it,” Djokovic said after his defeat to Medvedev. ”Just not a great match from my side. I thought I could have and should have done better, but credit to him for playing on a high level.”
Djokovic's long-time rival Rafael Nadal also has it all to play for in his final Group London 2020 match when the Spanish second seed takes on Stefanos Tsitsipas on Thursday evening.
Nadal, 34, has lost to Tsitsipas, seeded fifth this week, just once in six meetings — at the 2019 Madrid Open. He beat Tsitsipas here last year in the round robin before the 22-year-old Greek went on to win the title.
Nadal has never won the ATP Finals.
Also Thursday, Dominic Thiem, who has already qualified for the semifinals, faces Andrey Rublev. The Russian is 0-2 and can't advance.