Russia's Daniil Medvedev stunned world number one Novak Djokovic in straight sets to book a place in the last four at the ATP Finals.
Medvedev beat Djokovic 6-3, 6-3 in London's empty O2 Arena.
Meanwhile, in another Group Tokyo 1970 match, former champion Alexander Zverev got his campaign up and running.
He overcame Diego Schwartzman 6-3, 4-6, 6-3.
Here are further details.
A near-flawless defensive display by Medvedev shut Djokovic down.
He earned early break points and eventually made one count in the seventh game, which lasted 11 minutes.
The 24-year-old held serve in the next game and seized the opening set 6-3.
Medvedev then broke Djokovic again at the start of the second set to win his sixth game in a row and march ahead.
Medvedev lauded one of tennis' greatest champion Djokovic.
"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."
2018 champion Zverev struggled to find early rhythm against Schwartzman.
He was broken by Schwartzman in the third game but turned the tables with two breaks of his own to take the set.
Zverev stumbled twice on serve and conceded the second set.
In the fifth game of the decider Zverev broke with a fine backhand volley, before sealing victory.
Djokovic can progress into the semi-finals with a win over Zverev in their upcoming clash. Zverev is wary of the challenge ahead of him. "I'm excited for Friday," said Zverev. "Friday's going to be the most difficult match-up that you can have here, against Novak."
Love Sports news?
Subscribe to stay updated.