LONDON, Nov. 16 -- Eighth seed Jack Sock upset world No. 3 Alexander Zverev to enter the last four of the ATP Finals at the O2 Arena here on Thursday.

Sock, who qualified for the season-ending finale for the first time by winning the Paris Master two weeks ago, took the first set 6-4 but suddenly disappeared in the second, losing it in only 28 minutes. The 25-year-old American was broken in the first game of the decider and received a point penalty for smashing a ball off the court.

But Sock, the first American to reach the final weekend of the season since Andy Roddick in 2007, broke back and regained the momentum, sealing the victory 6-4 as Zverev made too many unforced errors.

"That was a roller coaster match, for sure," said Sock, who finishes second in Group Boris Becker, behind group winner Roger Federer.

He will face Grigor Dimitrov, who won Group Pete Sampras, on Saturday.

Earlier, Federer won his third consecutive group match after fighting back from one set down to beat fifth seed Marin Cilic.

The second seed needed one hour and 54 minutes to win 6-7 (5), 6-4, 6-1 to finish Group Boris Becker play 3-0. It's the 10th time Federer has ended group play unbeaten.

The six-time ATP Finals champion thus extended his unbeaten run to 13 matches, including eight indoors, dating back to his Swiss Indoors Basel title run.

The 19-time Grand Slam champion will take on either David Goffin or Dominic Thiem from Group Pete Sampras, where Grigor Dimitrov has secured the first place.

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