Serbia's Novak Djokovic celebrates after a winning point as he plays Russia's Andrey Rublev during their ATP World Tour Finals singles tennis match, at the Pala Alpitour in Turin on Nov 17, 2021. (LUCA BRUNO / AP)

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 Greek world number four was replaced in Turin by British world number 12 Cameron Norrie, who took Casper Ruud to three sets on his debut in the late match before the Norwegian prevailed 1-6 6-3 6-4.

Djokovic, who won his opening match against world number eight Ruud, was in early trouble in his first career meeting with Rublev when he was broken in the first game.

Russia's Andrey Rublev serves a ball to Serbia's Novak Djokovic during their ATP World Tour Finals singles tennis match, at the Pala Alpitour in Turin on Nov 17, 2021. (LUCA BRUNO / AP)

The good thing is that I qualified for semi-finals, so I won't have that much pressure to win the match... but obviously every match matters, so I will go with the intention to win.

Novak Djokovic, Serbian professional tennis player

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 taking 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.

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Djokovic will play Norrie on Friday while Rublev and Ruud meet in what amounts to a straight shootout for the last semi-final place.

"The good thing is that I qualified for semi-finals, so I won't have that much pressure to win the match... but obviously every match matters, so I will go with the intention to win," said Djokovic.

Serbia's Novak Djokovic returns the ball to Russia's Andrey Rublev during their ATP World Tour Finals singles tennis match, at the Pala Alpitour in Turin on Nov 17, 2021. (LUCA BRUNO / AP)

Ruud allowed the first set to get away from him before coming back strongly but Norrie still saved three match points in the final game to come back from 40-0 before the Norwegian served out.

Novak Djokovic will play Cameron Norrie on Friday while Andrey Rublev and Casper Ruud meet in what amounts to a straight shootout for the last semi-final place

"I wasn't really getting the feel of his shots in the beginning and I made too many mistakes," said Ruud. "I think for the first set and half the second he was the much better player out there. My serve saved me a little bit today."

The Norwegian said he had also been thrown by the late change of opponent.

"I woke up prepared for playing Stefanos and knowing I was the underdog and I could play freely and then you hear that he's pulling out... so then kind of everything changed," he explained.

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"I was feeling under pressure a little bit more, this was an opportunity for me to get my first win and the last time we played I beat him (Norrie)... and then the week after he went on and won Indian Wells."

Norrie was the second alternate drafted in following Italian Matteo Berrettini's withdrawal on Tuesday with compatriot Jannick Sinner taking his place.