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."