<p>Defending champion Daniil Medvedev recovered from a set down to defeat Poland's Hubert Hurkacz on Sunday on the opening day of the ATP Finals in Turin but said he had felt "zero pressure".</p>.<p>The Russian second seed triumphed 6-7 (5/7), 6-3, 6-4 over seventh-seeded Hurkacz.</p>.<p>"I had zero break points to save, so in a way I was never under pressure," said US Open champion Medvedev who fired 15 aces in the match.</p>.<p>"I had only two games on Hubert's serve where I had break points. Super happy to make this work and that I managed to get both breaks."</p>.<p>Hurkacz has enjoyed a breakout season, winning his first Masters event in Miami and making the semi-finals at Wimbledon after beating Roger Federer in the last-eight.</p>.<p>He is appearing in the end-of-season showpiece for the first time.</p>.<p>"I'll just try to stay positive," he said.</p>.<p>"I lost this match, but I'll think about the positives and what I can do better and prepare for the next one."</p>.<p>Medvedev has now won 23 of his last 26 matches.</p>.<p>Later Sunday, sixth-seeded Matteo Berrettini of Italy saw his challenge suffer a painful end when injury forced him to retire against Alexander Zverev.</p>.<p>The German third seed was 7-6 (9/7), 1-0 ahead when Wimbledon runner-up Berrettini called it quits with an apparent abdomen strain.</p>.<p>"I don't really know what to say, because this is the worst feeling a player can have," Zverev, the 2018 ATP Finals champion, said in his on-court interview.</p>.<p>"You play all year long to qualify for this beautiful tournament. For Matteo playing at home, this is the worst feeling I think he will ever have in his career."</p>.<p>Berrettini, who described his retirement as his "worst moment on a tennis court" has not yet pulled out of the event.</p>.<p>Should he decide to do so, then he would be replaced by another Italian in the shape of alternate, Jannik Sinner.</p>.<p>This year's ATP Finals are being played in Turin for the first time after being switched from London.</p>.<p>On Monday, world number one Novak Djokovic faces Casper Ruud while Stefanos Tsitsipas tackles Andrey Rublev.</p>
<p>Defending champion Daniil Medvedev recovered from a set down to defeat Poland's Hubert Hurkacz on Sunday on the opening day of the ATP Finals in Turin but said he had felt "zero pressure".</p>.<p>The Russian second seed triumphed 6-7 (5/7), 6-3, 6-4 over seventh-seeded Hurkacz.</p>.<p>"I had zero break points to save, so in a way I was never under pressure," said US Open champion Medvedev who fired 15 aces in the match.</p>.<p>"I had only two games on Hubert's serve where I had break points. Super happy to make this work and that I managed to get both breaks."</p>.<p>Hurkacz has enjoyed a breakout season, winning his first Masters event in Miami and making the semi-finals at Wimbledon after beating Roger Federer in the last-eight.</p>.<p>He is appearing in the end-of-season showpiece for the first time.</p>.<p>"I'll just try to stay positive," he said.</p>.<p>"I lost this match, but I'll think about the positives and what I can do better and prepare for the next one."</p>.<p>Medvedev has now won 23 of his last 26 matches.</p>.<p>Later Sunday, sixth-seeded Matteo Berrettini of Italy saw his challenge suffer a painful end when injury forced him to retire against Alexander Zverev.</p>.<p>The German third seed was 7-6 (9/7), 1-0 ahead when Wimbledon runner-up Berrettini called it quits with an apparent abdomen strain.</p>.<p>"I don't really know what to say, because this is the worst feeling a player can have," Zverev, the 2018 ATP Finals champion, said in his on-court interview.</p>.<p>"You play all year long to qualify for this beautiful tournament. For Matteo playing at home, this is the worst feeling I think he will ever have in his career."</p>.<p>Berrettini, who described his retirement as his "worst moment on a tennis court" has not yet pulled out of the event.</p>.<p>Should he decide to do so, then he would be replaced by another Italian in the shape of alternate, Jannik Sinner.</p>.<p>This year's ATP Finals are being played in Turin for the first time after being switched from London.</p>.<p>On Monday, world number one Novak Djokovic faces Casper Ruud while Stefanos Tsitsipas tackles Andrey Rublev.</p>