Novak Djokovic is once again red-hot favourite 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 number two -- behind Djokovic -- and the brightest young star in tennis.

Rafael Nadal, Djokovic's long-time rival, on Sunday 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 defence 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 number four another young pretender out to dethrone him.

It will be a tall order.

Djokovic has not lost at the Australian Open since 2018 and has a 20-0 record in semi-finals and finals at Melbourne Park.

"When I'm fit, when I'm at the peak of my performance, I can win any Slam or any tournament, I know that, I'm not afraid to say that," said Djokovic.

"It's not a secret that I want to break more records and make more history in the game."

Nadal farewell? 

Djokovic's haul of Grand Slam titles makes him the most successful player in the history of men's tennis, ahead of Nadal on 22.

If he wins again in Melbourne he will overtake the Australian Margaret Court, who won 24 majors in the women's game.

At least he no longer has to worry about Nadal, who has flown back to Spain for treatment.

The 37-year-old only made his comeback last week at the Brisbane International after being out of tennis since injuring his hip at the 2023 Australian Open.

Nadal's absence could spell the end of his career in Australia, after saying in Brisbane there was a "high percentage" that 2024 would be his farewell to the tour.

Medvedev threat 

Russia's Daniil Medvedev, a Melbourne finalist in 2021 and 2022, will also be a contender.

Like Alcaraz he chose not to play a warm-up event, but enjoyed a resurgent 2023 with 66 wins -– more than any other player on tour -- to earn five titles.

The 27-year-old, who won the US Open in 2021 for his only Slam crown, said he had never been more motivated.

"At this stage of my life, at least right now -- and I hope this can last for a long time -- I have just the biggest motivation ever to just continue finding my limits," he said.

Last year's losing finalist Tsitsipas will also fancy his chances, while world number five Andrey Rublev will get his campaign under way on the back of winning the Hong Kong Open.

The Australian Open starts on Sunday, becoming a 15-day event for the first time to help spread out the early round action and prevent some late-night finishes.