JOIN USJack Sock booked a place in the semifinals of the ATP Finals with a surprise 6-4, 1-6, 6-4 win over German prodigy Alexander Zverev on Thursday.
Sock is the first American to reach the semifinals 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 Roger Federer in the Boris Becker group and a Saturday semifinal date with Pete Sampras group winner Grigor Dimitrov.
"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.
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.
With Andy Murray, Novak Djokovic and Stan Wawrinka all absent due to injury and world number one Rafael Nadal withdrawing after his first match due to a knee problem, Federer appears to have a clear path to his seventh triumph in the season-ending event.
The prize money from Federer's win over Zverev on Tuesday took him past golfer Tiger Woods to become the world's highest earning athlete, with a career total of £84 million ($110 million, 94 million euros) according to Forbes.
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.