Novak Djokovic moved a step closer to retaining the Australian Open title with a straight-set semi-final win over injury-hit rival Roger Federer.

Serbia's Djokovic was 4-1 and 40-0 down in the opening set before recovering to win the tie-break.

That laid the platform for the 32-year-old second seed to go on and win 7-6 (7-1) 6-4 6-3.

Djokovic, aiming for a 17th Grand Slam title, will face Dominic Thiem or Alexander Zverev in Sunday's final.

"The match could have definitely gone a different way if he had used those break points [in the sixth game]," Djokovic said.

"He started well - I was nervous.

"Respect to Roger for coming out tonight. He was obviously hurt and wasn't close to his best in terms of movement."

Victory would mean a record-extending eighth Australian Open triumph for Djokovic, who has lost just three matches at Melbourne Park in the past 10 tournaments.

It would also move him within three of 38-year-old's Federer's all-time record of 20 men's Grand Slam singles titles and within two of Spain's Rafael Nadal, who lost to Thiem in Wednesday's quarter-final.

Ruthless Djokovic proves too strong

Djokovic was the heavy favourite to beat his long-time rival in what was their 50th meeting.

Questions were raised about Federer's fitness going into the match, having struggled with a groin problem in his epic quarter-final against Tennys Sandgren.

And he needed an off-court medical timeout, presumably for the same problem, at the end of the first set.

Djokovic turned into his ruthless best in the second set after a tight opener, giving away little on his serve and applying serious pressure on his opponent's.

After Federer survived break points in the second and sixth games, Djokovic took his first chance in the 10th, scampering to pick up a drop-shot and scoop away sharply cross-court.

That silenced many in the 15,000 crowd on Rod Laver Arena who were supporting Federer.

But not the noisy bunch of Serbs sat behind his player box, to where he shot a steely side-glance as he clenched a fist in celebration.

From that point it was difficult to envisage a comeback for Federer, who somehow came through five-set matches against third-round opponent John Millman and then Sandgren.

Djokovic picked his moment to break in the third set, finishing off a precise point with a wonderful forehand winner that kissed the sideline for a 4-2 lead.

Federer said after beating Sandgren he would always believe in miracles until defeat was confirmed.

This time it would truly have been a miraculous escape against someone of Djokovic's class.

Federer valiantly levelled at 30-30 as Djokovic tried to serve out the match, only for the Serb to refocus and seal victory in two hours 18 minutes.

Federer starts well but momentum swings

Concerns had been voiced by some at Melbourne Park that Federer might pull out in the hours leading up to the match. They proved unfounded.

Instead he came out firing in an extraordinary start in which his fitness issues did not stop him building up a lead.

Although he had to stave off two break points in the opening game, he eventually came through a six-minute hold before the pair exchanged three successive breaks.

Federer turned the set 4-1 in his favour with supreme serving and shot-making, leaving the stunned crowd murmuring in excitement at what they had just seen.

"It was not exactly the right mindset for me at the start - I was looking at how he was moving rather than executing my shots," Djokovic said.

Djokovic faced three more break points to go 5-1 down but hung on to leave Federer still needing two more games.

A 61-second hold to love - ace, backhand-volley winner, ace, service winner - put him within one.

Then, as the aces and winners started to dry up, momentum swung back as Djokovic won the next three games as a tie-break beckoned.

The pair had won 44 points each going into the set decider, despite Federer hitting 25 winners to Djokovic's six.

But Djokovic upped the ante in the breaker, blasting an ace past Federer for a 5-1 lead at the changeover and two more winners seeing him through.

That proved pivotal in the context of the match and ultimately terminal to Federer's chances.

"I managed to dig my way through and it was very important to win that first set," Djokovic said. "Then mentally I could relax and swing through the ball a bit more."

Race to be the 'GOAT' continues

Djokovic's victory over Federer in last year's historic Wimbledon final took him closer to the Swiss' Grand Slam tally than he has ever been.

Now the Serb, who has won six of the past nine tournaments in Melbourne, is aiming to further reduce the gap on Sunday.

Almost six years younger than Federer, Djokovic could add plenty more, barring a loss of form or fitness.

His pursuit of Federer and Nadal is made more remarkable by the fact he won his first major in 2008 - when Federer had claimed 13 and Nadal five - and only added a second three years later.

Nadal, however, is another major obstacle for Djokovic - and also a concern for Federer.