Cyril Thereau punished a wasteful Juventus with the only goal of the game as Udinese earned a 1-0 victory against the defending Serie A champions.
Thereau ghosted in at the back post with 12 minutes to play to fire Udinese to all three points in Stefano Colantuono's first league game at the helm.
After scoring 72 top-flight goals last season as they romped to a fourth successive title, Juve were uncharacteristically profligate on Sunday as they lost on home soil on the opening day for the first time.
Udinese had to ride their luck at times, with Orestis Karnezis' goal leading a charmed life as Mario Mandzukic and Roberto Pereyra went closest.
But it was at the other end that the breakthrough came as Thereau provided a close-range finish past an exposed Gianluigi Buffon.
Massimiliano Allegri used all three of his substitutions in the second half, but failed to get the desired response as Juve were stunned by a side that finished 16th last season.
Allegri's new-look Juve began life without Andrea Pirlo, Carlos Tevez and Arturo Vidal on the front foot, with Kingsley Coman particularly lively in attack alongside Mandzukic.
Midway through a half dominated by Juve, Pereyra was presented with the clearest opening after a mazy run from Coman, but the Argentine failed to get enough power in his shot to worry Karnezis.
Juve's failure to convert possession into goals nearly cost them on the half hour as Ali Adnan – the first Iraqi player in Serie A – jinked through two tackles on the edge of the penalty area, but his effort was the wrong side of Buffon's left-hand post.
Late pressure from the hosts saw Stephan Lichtsteiner sting the palms of Karnezis with a low drive from the right, before Mandzukic headed over a superb cross from the Swiss full-back.
Determined to get their campaign off to a positive start, Juve raised the tempo even further after the restart, but once again were let down by the quality of their finishing.
Mandzukic was unable to turn a diving header goalwards, Simone Padoin fluffed his lines 12 yards from goal after Coman's pass and Pereyra got the ball stuck under his feet five yards out in a frantic five minutes before the hour.
Thereau then stunned the Juventus Stadium with the game's only goal after 78 minutes, the unmarked Frenchman pouncing on a Panagiotis Kone cross to fire past Buffon.
Substitute Paulo Dybala led the charge on Udinese's goal in the closing minutes, but Juve's 47-game unbeaten streak at home in the league came to an end.
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