With the fitness of many of tennis' big stars questionable as the 2018 Australian Open comes around the corner, Roger Federer has backed Alexander Zverev to take advantage and have a breakout tournament.

With Andy Murray ruling himself out, and the likes of Novak Djokovic, Stan Wawrinka, and Rafael Nadal yet to prove they will be fit enough to go all the way, the defending Aussie Open champion believes world number 4 Zverev will be difficult to get past.

"I think every year that goes by he’s only going to become stronger at the Slam level," said Federer.

"He’s going to be very tough to beat here at the Australian Open. I think he’s doing all the right things.

"He had a great pre-season, he arrived early here in Perth. He’s put in some sacrifices."

Zverev has already earnt over $7 million in prize money in his short career so far, and reached the number 3 spot in the world rankings last November.

Nadal eliminated the 20-year old from last year's tournament in round 3 in a thrilling five-set match, but arguably Zverev's standout moment of the season was defeating the Serb Djokovic on clay in Rome.

"(It) was something I didn’t expect maybe," said Zverev. "Because it was on clay as well, a surface that other players had dominated for such a long time.

"I knew I could beat the best players but I’ve never really done it on clay. It was probably the most amazing tournament."

The Hamburg-born starlet has two Masters titles already to his name, and a Grand Slam is the next step on his rise to stardom, although he is keeping grounded on that possibility.

"The first time I came here [the Hopman Cup] I was 18-years-old, I was ranked 80 something in the world, now I’m the number four," Zverev stated.

"Things have changed a little bit but obviously I don’t want to stop here.

"I keep wanting to improve and keep wanting to get better because there is so much more I can accomplish."