Victory for the 21-year-old sees the Croatian secure only the second title of his career as he prevented Federer from capturing a record-extending 10th Halle victory.
Coric, who had reached the final without losing a set, maintained a high standard of grass-court tennis to beat the eight-time Wimbledon champion 7-6 (8-6) 3-6 6-2 after two hours and six minutes on court.
The opening stages of the showpiece saw both players provide compelling entertainment as Coric belied his relative inexperience to go toe-to-toe with Federer.
The Swiss great had won both their previous encounters but had been pushed to three sets at Indian Wells in March, and the world No 34 mixed heavy hitting from both wings and accurate serving to hold his serve and save the only break-point opportunity of the first set.
Federer, who had won in Stuttgart last week, appeared set to win the tiebreak after bringing up two set points, but Coric showed great strength of character to win four unanswered points and take the initiative in the contest.
Incredibly, Coric had only won two matches on grass coming into the week but continued to answer the questions Federer posed in the second set before the Croatian was eventually broken on Federer’s fourth break-point opportunity.
Federer closed out the second set with a consummate love-service hold to take the momentum into the decider.
Coric missed his first serve for the first time in the third set during the seventh game of the decider, but by this time had moved ahead after taking his first break-point opportunity of the match.
Despite the magnitude of the opportunity Coric held his nerve and finished the match with a flourish as he broke a stunned Federer for the second consecutive time to secure victory.
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