All had appeared well for the world No 1 in second match of the day, as he claimed the first set at a canter 4-2, but after appearing comfortable in the second, he succumbed to a 4-2 loss.
Krajinovic carried that momentum through to the decider, breaking twice but sealing the win in fortunate fashion, with a net cord return off the Djokovic serve.
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The loss came after Djokovic returned to the court in style on Saturday afternoon, sweeping past Viktor Troicki in straight sets.
The 17-time Grand Slam champion broke to love on match point to beat his compatriot and good friend 4-1, 4-1.
Watch the Adria Tour live on Eurosport
The second set was interrupted as Troicki handed over his racket to a ball kid - who promptly hit a fabulous cross-court forehand to take a point off the world number one.

Ball kid wins point against Djokovic with delightful drop shot at Adria Tour

Djokovic is the organiser of the event, which this weekend is taking place at the Belgrade tennis centre that bears his name.
The Adria Tour is an exhibition tournament that will travel around Serbia for the next three weeks as players compete against each other in group play.
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.

Damir Dzumhur

Image credit: Getty Images

Filip Krajinovic raced to a shock 4-0, 2-0 lead against a disappointing Alexander Zverev in the final match of the day session.
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