This is how Novak Djokovic's Australian Open came to an end.

As soon as Alexander Zverev claimed the first set, there was no doubt in the 37-year-old's mind that he had to retire from the match.

He immediately walked over to the net to shake hands with the second seed, before exiting with a wave to the crowd, who seemed to have mixed feelings about Djokovic's decision.

Image source, Getty Images

Image source, Getty Images

Image source, Getty Images

Image source, Getty Images

There will be no outright record 25th major title for Novak Djokovic in Melbourne this year.

But for Alexander Zverev, a first Australian Open final awaits against either defending champion Jannik Sinner or American Ben Shelton.

That one starts no earlier than 08:30 GMT.

This is just the second time Novak Djokovic has been forced to retire injured from the Australian Open, where he has won an astonishing 10 titles.

He had to call it quits in the quarter-finals in 2009 against Andy Roddick.

More from Alexander Zverev on Novak Djokovic:

"I'm happy on one side to be in the final of the Australian Open.

"On the other hand, I am being 100% honest, there is no guy I respect more than Novak.

"Whenever I have struggles I could always text him, whenever I needed advice, whenever I struggled mentally.

"He is always someone who has helped a lot. It is how it is. He has won this tournament 10 times and I have nothing but respect for him."

Here is what Alexander Zverev had to say after reaching the Australian Open final following Novak Djokovic's retirement:

"The very first thing I want to say is, please guys, don't boo a player when he goes out with injury.

"I know that everybody paid for tickets and everybody wants to see a great five set match.

"But you have to understand for the past 20 years Novak Djokovic has given tennis absolutely everything.

"He has won this tournament with an abdominal tear, a hamstring tear. If he cannot continue with this match, it means he really can't continue."

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Jonathan Jurejko
BBC Sport in Melbourne

Woah!! Not many - if anyone - saw that coming.

As soon as Djokovic lost an 81-minute set, he quickly put his thumb up and conceded to Zverev.

Even Zverev looked floored by what he was seeing.

There are some boos. Not cool.

Djokovic responds with a double thumbs up.

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There were audible boos for Novak Djokovic has he waved goodbye to the crowd on Rod Laver Arena.

Not what you want to see or hear.

Novak Djokovic is shaking hands.

We thought the first set could be crucial in this match, and Djokovic, clearly struggling with his left leg, says enough is enough after one hour and 21 minutes.

What a huge, huge shame.

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A huge miss from Novak Djokovic!

Alexander Zverev takes the first set.

Wait, what's this? It looks like Novak Djokovic can't go on...

But the first set point will go to Alexander Zverev!

Credit to the second seed, who is digging deep under pressure here and a closing forehand ensures he gets his reward.

Now, can he take this...

The players exchange aces to bring us level at four points apiece, before Novak Djokovic produces a sensational lob over Alexander Zverev to delight the crowd.

A show of class from the Serb. A psychological blow too? Djokovic hands over serving duties just two points away from the set.

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Jonathan Jurejko
BBC Sport in Melbourne

Only a fool would count out Novak Djokovic - I've done it many times.

But he does look in pain out there.

His walk to the chair at the change of ends is slow and laboured. The facial expression is often anguished.

An exquisite backhand volley from Alexander Zverev helps the German make a positive start to the tie-break after Novak Djokovic used his serve to take the opening point.

Zverev then holds his nerve with a forehand to break for the change of ends level at 3-3 after Djokovic fired back with a couple of points to maintain the pressure.

A sublime backhand down the line from Alexander Zverev helps the second seed on his way to a hold to love.

We're off to a tie-break!

And, given how this first set has gone, this feels like a very important one in determining the direction this semi-final will take.

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Alexander Zverev slices a backhand wide to relieve the pressure on Novak Djokovic after the German took the first point on his opponent's serve, and the 10-time champion settles another game from there to guarantee himself at least a tie-break.

Can Zverev keep responding when it really matters here?

How about that. In a first set defined by at times bizarre inconsistency from both men, Zverev records a love hold to put the pressure back on Novak Djokovic.

Image source, Getty Images

Jonathan Jurejko
BBC Sport in Melbourne

About to hit the hour mark and we're only nine games in. Nine!

No wonder the restless ones in the Laver crowd have felt the need to move.

Empty the bladder, refill the belly... and still maybe back in time for the next changeover.

Alexander Zverev has had his opportunities. Five of them, in fact.

But the German second seed has not been able to rise to the big points so far, and now he faces a really important game at the business end of this first set.

Impressive serving helps him make a positive start at 30-0.

With minimal backlift, Novak Djokovic knocks a clean backhand winner down the line for game point but then drops a forehand short to keep Alexander Zverev interested.

The former world number one again shows his quality with a forehand winner - and this time gets over the line with an energy-saving ace.

It has been a rather strange first set, but now Djokovic has it within his sights.

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Novak Djokovic knocks the ball up in the air in muted celebration after saving yet another break point.

Alexander Zverev swings his racquet in frustration after once again letting Djokovic off the hook.

The German is being far too passive in these big moments.