Stefanos Tsitsipas battled back from a set down against Dominic Thiem to win the ATP Finals title on Sunday, emphatically staking his claim as the leader of the next generation of aspiring global superstars. The Greek 21-year-old, who won, 6-7 (6/8), 6-2, 7-6 (7/4) is the youngest champion since Lleyton Hewitt in 2001 and the fourth first-time champion at the season-ending event in as many years. It is Tsitsipas’s third title of the season and comes a year after he was crowned champion at the Next Gen ATP Finals in Milan. He has...