The Russian comprehensively outplayed world number one Djokovic and at one stage won seven consecutive games, ultimately winning 6-3, 6-3.

Medvedev broke the Serbian three times, while holding serve throughout and producing nine aces compared to Djokovic's three.

With two wins from two, Medvedev is guaranteed to win the group, named Tokyo 1970.

Djokovic will have to beat Alexander Zverev is he is also to advance to the semi-finals.

Both players have a record of one win and one loss after German Zverev bounced back from his own defeat to Medvedev by winning versus Diego Schwartzman of Argentina.

Zverev triumphed 6-3, 4-6, 6-3 as Schwartzman was eliminated.

The winner of the Djokovic-Zverev match will play Austria's Dominic Thiem, who is already assured of topping the other group, London 2012.

In the doubles contest, top seeds Mate Pavić of Croatia and Brazilian Bruno Soares fell to Spaniard Marcel Granollers and Argentina's Horacio Zeballos, while Jürgen Melzer of Austria and Frenchman Édouard Roger-Vas saw off Australian pair Michael Venus and John Peers.

Competition at London's O2 Arena continues tomorrow.