Roger Federer has booked his place in the last four of the ATP Finals after beating Alexander Zverev.
Zverev is tipped to become world number one in the near future but on this occasion the master got the better of the apprentice as Federer battled to a 7-6 (8-6) 5-7 6-1 victory at the O2 Arena in London.
Federer is the first player to book his place in the semi-finals, while Zverev and American Jack Stock will also battle it out for a place in the last four in the final group match on Thursday.
Earlier eighth seed Sock beat Croatia's fifth seed Marin Cilic 5-7 6-2 7-6 (7-4) in the second round of matches in the Boris Becker Group.
"That was a tough one for sure," said Sock. "It's been an interesting morning so far, the fire alarm went off at 4am and we had to exit the building. But I love playing here in London, it's an amazing atmosphere, you make me feel like home.
"I'm just excited to win and keep myself alive."
Later today Grigor Dimitrov will play David Goffin in the afternoon, followed by Dominic Thiem against Pablo Carreno Busta, who replaces the injured Rafael Nadal, in the evening clash.