Sock downs Zverev to make ATP Tour Finals semis

London, United Kingdom: Jack Sock booked a place in the semi-finals of the ATP Finals with a surprise 6-4, 1-6, 6-4 win over German prodigy Alexander Zverev, while title favourite Roger Federer warmed up for the last four by subduing Marin Cilic on Thursday.

Sock is the first American to reach the semi-finals of the season-ending event since Andy Roddick in 2007.

The 25-year-old, up to a career-high ninth in the world rankings, only qualified for the Tour Finals thanks to his first Masters title in Paris earlier this month.

Sock was so sure he wouldn't make it to the O2 Arena that he had planned to play a round of golf at Augusta this week.

Instead of working on his putting, Sock ground out a hard-fought victory over third seed Zverev, securing second place behind Federer in the Boris Becker group and a Saturday semi-final date with Pete Sampras group winner Grigor Dimitrov.

Dimitrov, who takes on Pablo Carreno Busta on Friday, qualified for the last four after winning his first two matches.

Federer's opponent will be either Belgian David Goffin or Austria's Dominic Thiem, who clash in a winner-takes-all Sampras group tie on Friday.

"That was a rollercoaster match. I didn't have the hottest start in the third but I kept in there and kept fighting," Sock said.

Zverev is tipped as a possible heir to Federer's throne, but the 20-year-old still has work to do after an erratic display against Sock.

A bruising first set went in Sock's favour before Zverev took the second with ease.

When Zverev maintained his assault with a break in the first game of the deciding set, Sock smashed the ball into the crowd with such force that he was punished with a penalty point.

Sock channelled that rage with a burst of power hitting as he won four successive games.

In a dramatic finish, Zverev roared back from 4-1 down to draw level, only for Sock to pounce on his first match point.