An error-strewn Novak Djokovic was thoroughly tested Sunday by fearless teenage qualifier Dino Prizmic before launching his by bid for an 11th Australian Open and unprecedented 25th Grand Slam crown in unconvincing fashion.

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 No. 1 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.

Berrettini out injured

Fifth seed Andrey Rublev was hugely relieved after edging past Brazil's Thiago Seyboth Wild in a fifth-set tie-break.

Rublev had in his mind his good friend Daniil Medvedev's exit to Seyboth Wild at the same stage of the French Open last summer and he trailed 4-1 in the deciding tie-break after seeing a two-set lead and then four match points come and go.

But Rublev fought his way back to clinch a 7-5 6-4 3-6 4-6 7-6 (10/6) victory after three hours and 42 minutes.

Former Wimbledon finalist Matteo Berrettini was a casualty without taking to the court, the Italian pulling out as his injury woes continued.

Berrettini had been due to face Stefanos Tsitsipas on Rod Laver Arena on Monday in one of the plum ties of the first round.

But on Sunday afternoon tournament organizers announced he had withdrawn with a right foot injury, making it the fourth time in the last eight grand slam tournaments where he has not been able to play a match.

Berrettini has been replaced in the draw by Belgian lucky loser Zizou Bergs, who will take on Tsitsipas.

Fourth seed Jannik Sinner, one of the favorites for the title following his stellar end to 2023, began his campaign with a straight-sets win over Botic van de Zandschulp.

Sinner, who beat Djokovic twice in a matter of days at the ATP Finals and Davis Cup in November, was a 6-4 7-5 6-3 winner on Rod Laver Arena.

The Italian did not play a warm-up tournament after his late finish to last season and he said: "I feel like I started off actually really well for the first match.

Twelfth seed Taylor Fritz was twice a set down to Facundo Diaz Acosta and needed treatment after rolling his ankle but recovered to claim a 4-6 6-3 3-6 6-2 6-4 victory.

Former finalist Marin Cilic played his first grand slam match since the U.S. Open in 2022 following knee surgery but was beaten in four sets by Hungary's Fabian Marozsan.