Djokovic's win came before it was announced Britain's Cameron Norrie will make his debut at the ATP Finals against Norway's Casper Ruud after Stefanos Tsitsipas of Greece became the second player of the competition to withdraw through injury.

Italy's Matteo Berrettini was unable to recover in time for his group match against Poland's Hubert Hurkacz yesterday, with compatriot Jannik Sinner earning a straight-sets win in his place.

Tsitsipas suffered a right elbow injury.

Djokovic, meanwhile, won in serene fashion against Rublev, breaking four times in a 6-3, 6-2 victory.

The Serb, who secured his 50th win of the season and top spot in the group, smashed down a phenomenal 14 aces, won north of 80 per cent of his first-serve points and 60 per cent of the points on Rublev's second serve, as his returning skills again came to the fore.

Norrie then made his ATP Finals debut against Ruud, falling in three sets. 

The Briton won the first set 6-1 in sensational fashion, before dropping the final two sets 3-6, 4-6.

Ruud's victory means his third group-stage match against Rublev will determine who joins Djokovic in the semi-finals from the group.

Competition continues tomorrow with the conclusion of the Green Group, which Russia's Daniil Medvedev has already won with two wins in his first two matches. 

Each of the remaining trio of players can secure qualification, with only Germany's Alexander Zverev possessing his own fate.