London, Nov 15 : Dominic Thiem of Austria defeated Stefanos Tsitsipas of Greece in the first match of the ATP Finals tournament here on Sunday. The Austrian tennis player, who is ranked No. 3 in the world, won 7-6(5), 4-6, 6-3 against the world No. 6. Thiem hit nine aces against Tsitsipas's eight in the two-hour 17-minute match. This was a revenge match for Thiem who had lost to his Greek opponent in the final of the tournament last year. After the first two sets were closely fought, the US Open champion raised his game at the start of the third set to subdue Tsitsipas. "I experienced in the past four years how important it is to have a good start in this tournament, to ideally win the first match. I am very happy that I did it," said Thiem in after the game. Tsitsipas is the first defending champion to lose his opening match at this event since Roger Federer in 2008. "He dealt with my serve better and took time to step back and apply more pressure to me some of the second serves that I had early in the third set. I think that paid off for him," said Tsitsipas after the game. World No. 2 Rafael Nadal opens with Andrey Rublev of Russia later in the day while Novak Djokovic plays David Schwartzman on Monday. /IANS