Nov 17 (Reuters) - Novak Djokovic secured a semi-final place at the ATP Finals with a 6-3 6-2 victory over Russian Andrey Rublev on Wednesday, his 50th match win of the year, while Stefanos Tsitsipas withdrew from the tournament due to an elbow injury. The ATP said the Greek world number four was struggling withan elbow injury and would be replaced in Turin by British world number 12 Cameron Norrie. Djokovic, who won his opening match against Casper Ruud, was in early trouble in his first career meeting with Rublev when he was broken in the first game. The Serb, looking for his sixth title at the season-ending event, was given an immediate reprieve when Rublev double- faulted to bring up two break points, the second of which Djokovic converted. The pair were on an even keel until the eighth game when Djokovic pounced on a heavy volley from Rublev and sent a winning pass beyond the Russian to claim the vital break on his way to claiming the first set. The second was more assured from the world number one, forcing his opponent into errors that helped Djokovic to victory in 68 minutes, his 40th win at the ATP Finals. The 34-year-old and 20-times Grand Slam winner tops the Green Group standings with two wins from two and is assured of a place in the last four. Norrie will make his debut in the tournament later on Wednesday against Norway"s Ruud before facing Djokovic on Friday. "I always like playing new players, particularly in one of the greatest tournaments in the world here in Turin. Cameron deserves to be here, it’s not lucky," Djokovic said. "He worked his way to Turin with great wins in Indian Wells and some other tournaments, so it will be fun to watch him tonight against Ruud and I’ll get ready for my next match." Norrie is the second alternate drafted in following Italian Matteo Berrettini"s withdrawal on Tuesday with compatriot Jannick Sinner taking his place.