Jack Sock delivered a stunning performance on Thursday at the ATP Finals, defeating third seed Alexander Zverev 2-1 to secure a spot in the semifinals. With two wins from three group-stage matches, the American advanced alongside six-time champion Roger Federer from Group B, turning what seemed like a long shot into a career-defining moment. At 25 years old, Sock's 2023 season has been nothing short of miraculous. Just two weeks before the tournament, his ATP ranking stood at a modest No. 22, placing him well outside contention for the year-end…