<p class="bodytext">Jack 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.</p>.<p>Sock is the first American to reach the semifinals of the season-ending event since Andy Roddick in 2007.</p>.<p class="bodytext">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.</p>.<p class="bodytext">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.</p>.<p class="bodytext">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.</p>.<p class="bodytext">"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.</p>.<p class="bodytext">Dimitrov, who takes on Pablo Carreno Busta on Friday, qualified for the last four after winning his first two matches.</p>.<p class="bodytext">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.</p>.<p class="bodytext">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.</p>.<p>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.</p>.<p class="bodytext">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.</p>.<p class="bodytext">A bruising first set went in Sock's favour before Zverev took the second with ease.</p>.<p class="bodytext">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.</p>.<p class="bodytext">Sock channelled that rage with a burst of power hitting as he won four successive games.</p>.<p class="bodytext">In a dramatic finish, Zverev roared back from 4-1 down to draw level, only for Sock to pounce on his first match point.</p>
<p class="bodytext">Jack 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.</p>.<p>Sock is the first American to reach the semifinals of the season-ending event since Andy Roddick in 2007.</p>.<p class="bodytext">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.</p>.<p class="bodytext">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.</p>.<p class="bodytext">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.</p>.<p class="bodytext">"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.</p>.<p class="bodytext">Dimitrov, who takes on Pablo Carreno Busta on Friday, qualified for the last four after winning his first two matches.</p>.<p class="bodytext">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.</p>.<p class="bodytext">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.</p>.<p>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.</p>.<p class="bodytext">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.</p>.<p class="bodytext">A bruising first set went in Sock's favour before Zverev took the second with ease.</p>.<p class="bodytext">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.</p>.<p class="bodytext">Sock channelled that rage with a burst of power hitting as he won four successive games.</p>.<p class="bodytext">In a dramatic finish, Zverev roared back from 4-1 down to draw level, only for Sock to pounce on his first match point.</p>