Despite his lesser billing, the eighth seed claimed the opener 6-4 and ultimately capitalised on Zverev’s mental fragility to win a tense final set 6-4. The German had swept through the second set 6-1.
“It was a roller coaster final set,” said Sock. “I was a break down then was able to break back and then got another one and then gave one away and then turned it round.”
Sock sealed the decider when Zverev fluffed his lines to level at 5-5. “I choked,” said Zverev. “It’s that simple. At 4-5 down, that was one of the worst games I think I played all year. So, yeah, I just choked. Nerves got in the way and got the better of me.”
Sock is the first American since Andy Roddick in 2007 to reach the semi-finals at the end of season championship. And his progress defies expectations.
On the opening day of the event on 12 November, the script went according to plan. Sock lost his opening match in the Boris Becker group to second seed Roger Federer. But two days later Sock went rogue. He saw off fifth seed Marin Cilic in three sets and the victory over Zverev, who is also making his debut at the tournament, was a shock.
“The guy is 20-years-old and has played some absolutely outstanding tennis this year,” said Sock of his opponent. “He cannot legally drink a beer in the United States and he is number three in the world.
“I was down 5-1 in the final set in the second round in Paris,” Sock recalled. “I came back and won and so I’ve been playing with ‘house money’ ever since. I’m going out there letting loose and having fun and trying to put on a show a little bit and enjoy my time.”
Sock’s irreverence has illuminated the tournament. He crept up behind Federer while the Swiss star was reviewing his match against Cilic for a British TV station. And he has tried unorthodox shots during his games.
“Obviously when you sneak into the eighth and final spot for the tournament a week before, it definitely takes some nerves away. I’m hitting some stupid shots that work sometimes. Qualifying late is helping me.”
Sock, who will play Grigor Dimitrov in Saturday’s semi-final for a place in Sunday’s final, has won three of their four encounters. Their last meeting came in the last 32 at the Indian Wells Masters in March when Sock saved four match points to advance.
“I think Grigor’s a little bit of a different player since the last time we played,” said Sock. “I think he has found his game and what he wants to do when he is on a tennis court. It’s been showing throughout the year. He is playing deep into tournaments every week and has really established himself in the top 10.”
He added: “But I’m playing with confidence coming off the back of Paris. I’m playing here for the first time and I’ve won some matches to get into the semis. I’m proud of myself and looking forward to getting out there and competing.”