German world No 3 battled back to win the second set and lead by a break in the decider before losing to his American opponent.
'I choked' insists Zverev as Sock joins Federer in ATP Finals last-four
Alexander Zverev has enjoyed a sensational year, winning five tournaments and reaching world No 3, but it all ended in anti-climax on Thursday at the ATP Finals.
Zverev lost his final round-robin match 6-4, 1-6, 6-4 to American Jack Sock who moved into the semi-finals at the expense of the man tipped to be a future grand slam champion.
When Zverev, 20, won the second set and broke at the start of the third against the world No 9, it seemed just a matter of time before he was powering smoothly to victory.
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But the German then completely lost his focus to go 4-1 down and although he battled back to 4-4, he produced a horror of a game at 4-5, serving an eighth double fault at 30-30 before a lame forehand into the tramlines ended the contest.
"I choked. It's quite easy. Won the second set 6-1. I was 1-0 with a break. He got a point penalty. I was down 1-4 within 10 minutes where I didn't put many balls in the court," he told reporters. "When I got back at 4-5, that's one of the worst games I think I played all year."
Asked to reflect on his breakthrough season, Zverev found it hard to look past a disappointing finish.
"It's been an awesome year. But the end of the year was [not good] for me," said Zverev, who marked his first ATP Finals qualification with a win over Marin Cilic before losing a three-setter to six-times champion Roger Federer.
"If I would have played the whole year like I did, by the end of the year I don't think I would have finished top 50. That's a bit unfortunate for me."
Zverev said he will not stew over it for long though.
"I'm going to go on holidays now. I'm going to enjoy my brother's wedding. I'm going to do that in the Maldives. I'll get over it. I'm definitely going to enjoy that.
"Then I'm going to work hard in the off-season."