Jack Sock became the first American to reach the last four of the ATP Finals for 10 years when he stunned world number three Alexander Zverev in a firecracker of a match on Thursday.
The world number nine, who barged through the back door to qualify for the London showpiece by winning the Paris Masters, triumphed 6-4 1-6 6-4 to prolong his London adventure.
Roger Federer had already won the Boris Becker group, having battled back to beat Marin Cilic earlier to post a perfect round-robin record at the event for a 10th time.
Six-times champion Federer won 6-7(5) 6-4 6-1 leaving the stage set for Zverev and Sock to slug it out for the runners-up spot and a semi-final against Bulgarian Grigor Dimitrov.
The 25-year-old from Kansas looked to have blown himself out when a 4-1 lead evaporated in the decider, but the 20-year-old Zverev admitted later he “choked” when serving at 4-5, double-faulting and then bunting a forehand wide to hand Sock victory.
“That was a rollercoaster of a match for sure, that’s been the theme of my last few weeks, hang in there and keep fighting,” Sock, who seemed to be imploding when he received a penalty point at the start of the third set, said on court.
Andy Roddick was the last American to reach the semis.
His opponent in the semifinals will be fellow debutant, Grigor Dimitrov. The Bulgarian has also been in stellar form so far and this match will be hotly anticipated. The American leads the head-to-head 3-1 so there is a massive opportunity for Sock to make the final. The Paris champion leads 2-1 on hard courts and each of these three meetings went the distance, so an epic could be on the cards here in London. A title here could see Sock climb as high as #4 in the World, and the American sees a huge chance to make this happen.
