Novak Djokovic secured a semifinal 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 world No.1, bidding to equal Roger Federer's record of six titles at the season-ending event wrapped up top spot in the Green Group thanks to successive straight-sets victories. 

"I served well (and) that helped tremendously," Djokovic said. 

"I wanted to put him out of his comfort zone, taking away the time and mix up the pace. It was a great performance overall." 

The 34-year-old Djokovic is looking to cap off a remarkable year which saw him only just miss out on completing a calendar Grand Slam with defeat by Daniil Medvedev in the U.S. Open final. 

A rematch with second-ranked Medvedev in the final in Italy is still a possibility, with the Russian reigning champion sitting top of the Red Group. 

Djokovic is bidding for back-to-back titles after returning to action earlier this month with success in the Paris Masters, beating Medvedev in the final. 

The Serb has not won the ATP Finals since 2015, losing finals in 2016 and 2018 to Andy Murray and Alexander Zverev respectively, while also falling in an epic last-four clash with Dominic Thiem last year. 

Norway's Casper Ruud beat ATP Finals debutant Briton Cameron Norrie 1-6 6-3 6-4. Norrie replaced Greek star Stefanos Tsitsipas who pulled out with an elbow injury earlier. 

Italian youngster Jannik Sinner is in a similar position in the other group, after replacing the injured Matteo Berrettini and then seeing off Hubert Hurkacz (6-2, 6-2) on Tuesday. 

Meanwhile, Djokovic said on Wednesday he hopes rival Roger Federer will return to competitive action soon and be able to compete with the best again after the Swiss great said he would miss the Australian Open in January. 

Federer, 40, told Swiss media he is unlikely to return to competition until mid-2022 as he recovers from multiple knee surgeries and would be "extremely surprised" if he was able to play at Wimbledon, a Grand Slam he has won a record eight times. 

(With input from agencies)