Novak Djokovic says world no. 1 Carlos Alcaraz's withdrawal from the Australian Open is 'not good for tennis', though the Serb's chances at a tenth title are hugely boosted by his now decidedly cruisy draw.

The young Spanish superstar, who became the youngest ever player to reach no. 1 in the world after winning the US Open in September, withdrew from the upcoming tournament in Melbourne after suffering a hamstring injury during training

It gives the Serbian great a huge boost as he looks to earn a record-equalling 22nd Grand Slam, not that he was focused on that when speaking about Alcaraz's untimely withdrawal in the wake of a raft of missing stars.

'Obviously, it's not good for tournament, not good for tennis,' said Djokovic on Saturday following his Adelaide International semi-final win, when asked about the 19-year-old's withdrawal from upcoming Open.

'He's number one in the world. Such a young player. A big name, big star in our sport already. I'm sure that many of the fans are disappointed. I hope for his speedy recovery.' 

The Serb, who is gunning for his tenth Australian Open title after first winning in 2008, said the gruelling nature of the ATP tour meant injuries were an unfortunately reality for players.  

'Whatever it is, I know he had injury end of last season that was different from this,' he said.

'You're constantly on the road, playing competitions. When you are out of that rhythm, your body can react.

'You can have some injuries and difficulties when you come back to the tour and start pushing more and playing points, giving more intensity in the points, practice points, practice matches.

'It's unfortunate for him (but) he's still young, got plenty of time. I hope he can recover and be back on the tour quickly,' said a hopeful Djokovic, who has been incredibly popular with Aussie fans since arriving Down Under for the Adelaide International, his Open preparation.

Alcaraz became the youngest man ever to reach the top spot after beating Casper Ruud in the US Open final in New York in September.

The superstar teen had only just recovered from a torn abdominal muscle that ruled him out of the season-ending ATP Finals before the hamstring injury in a preseason training session meant he had to pull out of the storied Melbourne event.

But Daniil Medvedev, who was succeeded by Alcaraz as world No.1, had an encouraging message for the teen, tipping him to bounce back quickly.

'It's a pity. The way he (Alacaraz) played at 18 years old, he could be a little bit kind of Rafa, Novak, Roger kind of guy,' said the Russian, who has played in the last two Australian Open finals, without a win.

'But he still has 19 slams to win, so it's a pity that he doesn't play the next one. Would be fun to watch. I'm sure he's going to come back stronger.'

It is absolutely a blow for Aussie tennis fans in the wake of a spate of high-profile withdrawals and injury concerns.

Of course there's also the absence of the now retired superstar trio: Ash Barty, Serena Williams and Roger Federer. 

What it does mean, however, is that the fan favourite Djokovic has now dramatically firmed into almost unbackable favouritism with the bookies.

That's despite last year's champion Rafael Nadal being the top seed, though the ageing superstar is stuck in a rut of continual poor form as he enters the end of his illustious career.

Thanks to Alcaraz's withdrawal, Djokovic is now the fourth seed for the tournament, which begins on January 16. 

The elevation means the 35-year-old is guaranteed to avoid playing a higher-ranked rival - Nadal, Ruud or Stefanos Tsitsipas - until at least the semi-finals. 

As a result, Djokovic has firmed strongly in TAB.com.au market to win the tournament, with the Serb sitting at $1.95, with Medvedev the next favourite at $5.50.

Aussie Nick Kyrgios - who has never enjoyed success in the singles of his home Grand Slam - is the third favourite, alongside Nadal ($12); while Felix Auger Aliassime ($14), Tsitsipas and Jannik Sinner ($17) and Holger Rune ($21) sit well behind. 

It has been a remarkable turnaround for Djokovic after the deportation saga rocked last year's Australian Open. 

First he successfully had his three-year visa ban overturned in November, allowing the nine-time champion to return to Melbourne to extending his own record of most Australian Open titles.

Now his chief challenger is out of the tournament and Djokovic - who this week extended his five-year unbeaten run in Australia to 32 matches in Adelaide - will be visualising lifting the Norman Brookes Challenge Cup on Rod Laver Arena yet again come January 29.