Jack Sock seals semi-final spot as Alexander Zverev crashes out

Sock took an opening set that saw Zverev play below his best, but the young German stormed back in incredible fashion to take the second set 6-1.

However Sock wasn't rattled and the American dug deep to break in the third and seal a win that confirms his spot in the semi-finals.

"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.

Sock will face the winner of the Pete Sampras group.

Federer had already booked a semi-final spot with his earlier win over Zverev but any notion that he would coast through his final group match proved wide of the mark.

Cilic was hoping to end his year on a high in the city where his Wimbledon dream was ended in this year's final by Federer with the Croatian sobbing on court after suffering blisters.

The world number five put up much more resistance this time, winning the first set, and when Federer served at 4-4, 30-30 in the second it seemed Cilic might enjoy some redemption.

But the Swiss simply hates to lose and when Cilic netted a forehand he pounced to square the match.

A 100mph forehand service return gave Federer an early break in the decider and he turned on the style to race to victory.

Federer has reached the last four in 14 out of 15 appearances in the year-end tournament and will face David Goffin or Dominic Thiem for a place in the final.