An error-strewn Novak Djokovic kickstarted his bid for an 11th Australian Open and unprecedented 25th Grand Slam crown in unconvincing fashion on Sunday, dropping a set to fearless teenage qualifier Dino Prizmic.
The Serbian superstar struggled to impose himself against the confident Croat on Rod Laver Arena before finally taming the 18-year-old 6-2, 6-7 (5/7), 6-3, 6-4 after a four-hour marathon.
While far from his best, the world number one showed no sign of the right wrist problem that hampered his build-up.
Djokovic surged to the title last year with a three-set victory over this year's seventh seed Stefanos Tsitsipas.
If he does the same again, he will pass Australian Margaret Court's all-time Slam record and join only Court (Australian Open) and Rafael Nadal (French Open) in winning 11 times at a single major.
Victory stretched his record at Melbourne Park to 90-8 as he reached the second round for a 17th consecutive year.
But he was troubled by Prizmic's speed and booming baseline play, which forced many uncharacteristic errors in a match many thought would be a cruise.
The Croat is the current French Open boys' champion and he used the occasion to showcase his talent, undaunted by a Grand Slam main draw debut on Djokovic's most successful court.
He lost his opening service game, but held his next two before needing a medical timeout for a left thigh problem.
When he returned, Djokovic mercilessly moved him around the court to take the first set in 42 minutes.
Undeterred, the Croat, with his leg strapped, raced to a 3-1 lead in the second set, breaking Djokovic after he was hit with a time violation then belting a glorious cross-court winner.
The Serb got back on level terms but it went to a tie-break, where Prizmic took charge, racing 6-2 clear. Djokovic saved three set points but not a fourth.
Djokovic pulled 2-0 clear in the third set, only for Prizmic to keep punching above his weight, winning the next three games.
But the defending champion found a new wind, saving two break points to hold for 4-3 before closing out the set, roaring in delight as he did so.
He broke the tiring teenager immediately in the fourth set before again needing to repel a fightback.
Battling to the end, Prizmic saved multiple match points before finally succumbing, with Djokovic next playing the winner of an all-Australian showdown between Marc Polmans and Alexei Popyrin.