The matches are best-of-three sets, with each set being first to four games.
Djokovic warms up by dancing with DJ and pianist before Adria Tour match
Grigor Dimitrov began the day with a win over Serbian Dusan Lajovic, winning 4-3(2), 3-4(5), 4-1.
Dimitrov wins point with cheeky tweener against Lajovic
It was a disappointing return for Damir Dzumhur, who pulled up with a leg injury after just two points of his match against world number three Dominic Thiem, and was forced to retire shortly afterwards.
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Filip Krajinovic raced to a shock 4-0, 2-0 lead against a disappointing Alexander Zverev in the final match of the day session.