Thiem took the first-set tie-break 6-2 to take an early lead - but Krajinovic got the first break of the match in Thiem's second service game of the second set.
The Serbian took a medical time-out in the middle of the second set for some treatment on his right arm - but it did not affect him too much, with Thiem double-faulting on set point to level matters up.
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The world number three gathered himself, though, and held serve to love twice in the deciding set.
Krajinovic clung on to his own serve - and forced Thiem to serve out to love to win the match.
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He got to match point in style, as well, hitting a fabulous tweener, much to everyone's delight - except perhaps Krajinovic.
The world number 32 was a surprise finalist, progressing on the percentage of games won - and pipping tournament creator Novak Djokovic to the occasion.
This was the first weekend of the Adria Tour, with the matches hosted at the tennis centre in Belgrade that bears the world number one's name.

Emotional Djokovic in tears as he leaves Adria Tour in Belgrade

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