Greek tennis sensation Stefanos Tsitsipas produced a terrific performance as he eased past 20-time Grand Slam champion Roger Federer in straight sets to book his place in the summit showdown of the Association for Tennis Professionals (ATP) Finals at O2 Arena in London. The 21-year-old hardly broke a sweat as he eased past Federer 6-3, 6-4 in the semi-final clash of the season-finale tournament that lasted a little more than one and a half hour. In the opening set, Swiss maestro Roger Federer made a poor start as he botched two smashes in his first game before Tsitsipas hit an angled forehand winner to take advantage. Subsequently, Federer wasted another three break points when the Greek served at 4-2 before Tsitsipas finally sealed the set on his seventh set point. Tsitsipas then broke Federer early in the second set before Federer hit back to level the score at 2-2. The Swiss, however, soon lost the break to go down 3-2. Federer failed to serve nearly as well against Tsitsipas as he did in the previous match against Novak Djokovic before the Greek hit back to finish an absorbing clash with an ace. Following the clash, Tsitsipas said that it was…