Jannik Sinner became the first teenager to win an ATP 500 event, beating Mackie McDonald 7-5, 4-6, 7-5 to take the Citi Open title.

Stefanos Tsitsipas of Greece drops his racquet as he wins and defeats Austria's Dominic Thiem in their ATP World Finals singles final tennis match at the O2 arena in London, Sunday, Nov. 17, 2019. (AP Photo/Kirsty Wigglesworth)LONDON – Three years after coming to the ATP Finals as a hitting partner to the top pros, Stefanos Tsitsipas is leaving the tournament as champion. Tsitsipas rallied to beat Dominic Thiem 6-7 (6), 6-2, 7-6 (4) on Sunday for the biggest title of his career, becoming the youngest champion at the season-ending event in 18 years. ”He's great for tennis, because he has a very attractive game style, one-handed backhand, comes in a lot,” Thiem said. ”That's why it's probably mentally the most brutal sport existing, because you can play such a great match and end up losing in the championship match,” Thiem said.