TURIN, Italy, Nov 16 (Reuters) - Defending ATP Finals champion Daniil Medvedev maintained his perfect start to this year"s tournament but found it hard going against Alexander Zverev, eventually winning 6-3 6-7(3) 7-6(6) in their Red Group clash on Tuesday. Medvedev had won his opening match against Hubert Hurkacz while Zverev was given a walkover in his opener on Sunday against Italian Matteo Berrettini, who retired injured and then withdrew from the tournament on Tuesday. The Russian world number two, who won his first Grand Slam this year at the U.S. Open, raced into a 3-0 lead in the first set by frequently coming to the net and looked like he would take a comfortable fifth straight victory over Zverev. However, the German found some confidence in Turin to bring himself back into the contest, firing down 13 aces and taking the second set on a tiebreak. The pair could not be separated in the decider with neither able to break serve and both soaking up some heavy hitting. In the decisive tiebreak, Zverev saved two match points but could only hit a backhand into the net on a third to give Medvedev his seventh straight win at the ATP Finals after two hours and 35 minutes. Zverev, 24, faces Poland"s Hurkacz in his final round-robin match looking to clinch a place in the last four. The Green Group resumes on Wednesday with Novak Djokovic facing Andrey Rublev and Stefanos Tsitsipas taking on Casper Ruud. Berrettini announced that he was withdrawing from the event in a post on Instagram after suffering an abdominal injury. "My finals end here, I"m destroyed, I never thought I"d have to give up the most important tennis event ever held in Italy in this way," he wrote. "The truth is that as much as I wanted to play in front of you once again, I felt and therefore decided that my body is not ready to face the challenges that lie ahead." The ATP said that first alternate Jannik Sinner will take compatriot Berrettini’s place.