Jack Sock qualified for the ATP Finals by winning the Paris Masters
ATP Finals
Venue: The 02 Arena, London Dates: 12-19 November
Coverage: Watch live on BBC Two, Red Button, BBC Sport website and mobile app, listen on BBC Radio 5 live sports extra and follow text updates online.
American Jack Sock came through a pulsating "rollercoaster match" to beat German third seed Alexander Zverev and reach the last four at the ATP Finals.
Eighth seed Sock won 6-4 1-6 6-4 to clinch second place in the Boris Becker Group ahead of Zverev, and set up a semi-final against Grigor Dimitrov.
Roger Federer earlier beat Marin Cilic 6-7 (5-7) 6-4 6-1 to head into the semi-finals with an unbeaten record.The Swiss, 36, had already topped the group with wins over Zverev and Sock.
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Saturday's semi-final line-up will be completed by the winner of the Pete Sampras Group match between Dominic Thiem and David Goffin at 14:00 GMT on Friday.
"Knowing I was qualified on Tuesday evening - it's kind of weird to be already through to the semis, but of course it gives me a great idea how I want to prepare for Saturday," said Federer.
"One more last weekend, one more push before the vacation and then preparation for the next season."