MELBOURNE, Australia (AFP) — Novak Djokovic is once again red-hot favorite at the Australian Open as he chases an unprecedented 25th Grand Slam title, but a wrist problem and the emergence of Carlos Alcaraz puts his all-conquering reign under threat.

The 36-year-old from Serbia defeated Stefanos Tsitsipas in last year's final for a record-extending 10th Melbourne crown then added the French and US Open titles to his collection in 2023.

But he lost a pulsating Wimbledon final to Alcaraz, who is a different player to the one that last graced the Australian hard courts as an 18-year-old ranked outside the top 30.

The 20-year-old missed last year's event with a hamstring injury and arrives this time as a two-time major champion, world No. 2 — behind Djokovic — and the brightest young star in tennis.

Rafael Nadal, Djokovic's long-time rival, pulled out of the Australian Open with a muscle tear in a fresh blow to the veteran's hopes of returning to the top of the sport.

Nadal's fellow Spaniard Alcaraz opted to skip a warm-up tournament for the opening Grand Slam of the year, making his 2024 form a mystery.

But Djokovic dropped a set to the Czech Republic's Jiri Lehecka at the United Cup and then crashed to Australian Alex de Minaur 6-4, 6-4.

It snapped a 43-match win streak in Australia and Djokovic will begin his defense with an injury cloud hanging over him, needing treatment on his right wrist through both matches.

"I think I have enough time to get myself in the right shape for the Australian Open and that's what matters the most at this point," he said after losing to De Minaur.

"It's all part of the build-up for the Australian Open. That's where I want to perform at my best."

Djokovic's United Cup troubles followed defeat to Jannik Sinner at the Davis Cup to end his 2023 season, with the burgeoning Italian world No. 4 another young pretender out to dethrone him.

It will be a tall order.