|ATP Finals 2020|
|Venue: O2 Arena, London Date: 15-22 November|
|Coverage: Watch live on BBC Two, BBC iPlayer, BBC Sport website and mobile app; follow BBC radio and live text commentary online on selected matches|
An error-prone Novak Djokovic was beaten for only the fourth time this year as Daniil Medvedev reached the semi-finals at the ATP Finals.
The Serb hit 24 unforced errors and five double faults in a 6-3 6-3 loss to fourth seed Medvedev in London.
World number one Djokovic, 33, can still qualify for the last four if he wins his final round-robin match against Alexander Zverev on Friday.
Zverev earlier boosted his own hopes with victory over Diego Schwartzman.
The German fifth seed overcame a poor second set to claim a 6-3 4-6 6-3 win against the Argentine, who is now unable to reach the knockout stage after Medvedev's win at the O2 Arena.
Djokovic has no answer to Medvedev's energy
Djokovic came into this match having lost just three times in 2020 - when he was defaulted at the US Open, when he was beaten by Rafael Nadal in the French Open final and then when he was upset by Lorenzo Sonego in the quarter-finals in Vienna.
He is chasing a record-equalling sixth title here and although he still has a good chance of advancing, he must do it the hard way against Zverev.
The Serb was made to work hard from the off by Medvedev, who created two break points in the third game of the opening set in a high-energy display.
Despite some lengthy games, including one that lasted seven minutes and another that went to five deuces, Medvedev did not tire as he broke for 4-3 when Djokovic netted a forehand.
A double fault handed the 24-year-old Russian the set as part of a run of seven games in a row that he strung together to lead 3-0 in the second.
Djokovic got himself on the scoreboard but it was too late as Medvedev continued to dominate the baseline rallies and went 5-2 up.
The Serb then held to force the Russian to serve it out, which he did with confidence as a forehand winner wrapped up his second victory in two matches in the Group Tokyo 1970.
"I cannot allow these things [losing seven games in a row] to happen when I'm playing the top players in the world," Djokovic said.
"He was just better, no question about it. We started well. Up to 3-3 [in the first set] it was quite an even match but after that break he was cruising. I struggled to find the right level."
Medvedev, who made only 12 unforced errors and won 10 of the last 13 games, is experiencing a very different ATP Finals to last year where he lost all three of his group matches on his debut.
"I'm sure he didn't play his best but it happens to everybody," Medvedev, who advances as group winner, said in an on-court interview.
"I'm feeling confident. I knew I had to take my chances and it was a great match for me."
Who can qualify for the semi-finals?
The permutations at this event are often mind-bendingly complicated but this year it is straightforward.
With the two group winners already decided after Dominic Thiem clinched Group London 2020 with victory over Nadal on Tuesday, there are two winner-takes-all matches to determine the remaining semi-finalists.
Before Djokovic and Zverev meet on Friday, 20-time Grand Slam champion Nadal and defending champion Stefanos Tsitsipas face each other on Thursday (20:00 GMT).
In Thursday's other match US Open champion Thiem takes on already eliminated Russian Andrey Rublev (14:00).
Meanwhile, Britain's only representative at the event - Joe Salisbury - and his American partner Rajeev Ram will be chasing a place in the doubles semi-finals on Thursday. They need to beat German pair Kevin Krawietz and Andreas Mies to advance (18:00).
BBC tennis correspondent Russell Fuller
This was only Djokovic's fourth defeat of the season - and was all the more surprising given the way he started the match.
Medvedev did very well to weather the storm, and then improbably and superbly to reel off seven games in a row.
Eight second-set aces tightened his grip on the match, but there was a curious indifference to Djokovic as the tide turned against him.
The round-robin format can play tricks with the mind: Djokovic knew he could not be knocked out tonight.
But now he must beat Zverev on Friday afternoon. Will he be able to rouse himself with a place in the semi-finals on the line?
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