With Serbia needing a win after Russia's Andrey Rublev breezed past Filip Krajinovic earlier in the day, it was a relatively straightforward victory for Djokovic, who never really looked like slipping up as he surged into a 4-1 lead before taking the first set 6-3.
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The second set was a tighter affair initially with the scores level at 3-3, but it was Djokovic who broke away - on a day where his service game impressed (11 aces) - and emerged victorious.
The result means the quarter-final tie will go to a decider with Djokovic and Viktor Troicki taking on Rublev and Khachanov.


Rublev stormed to an impressive 6-1 6-2 victory over Krajinovic to initially give Russia the edge.
After a brutally efficient opening service game, Rublev followed it up perfectly with a break to love as he raced into a 5-0 lead against a shell-shocked Krajinovic.
Rublev wrapped up a routine opening set and then set about continuing in the same vein in the second, going 4-0 up.
Krajinovic managed to get two more games on the board but he was powerless to stop his opponent's march to victory.
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