Image source, Getty Images

Carlos Alcaraz has a smile back on his face after he whips a forehand past Alexander Zverev. Another good sign.

That temporarily disappears when a double fault brings Zverev to 30-30, but Alcaraz beats the third seed with a perfectly weighted drop shot and then overpowers him with a backhand down the line.

The Spaniard calls for more noise from the crowd. He's finding his rhythm again.

Jonathan Jurejko
BBC Sport tennis news reporter in Melbourne

My Spanish listening skills are not completely up to scratch, I'll be honest.

But it is good enough to hear what Samu Lopez has been telling Alcaraz.

"Keep fighting" and "continue" has been the gist of the message.

The Alcaraz box is hoping the pickle juice will get working as quick as it is supposed to.

Image source, Reuters

Carlos Alcaraz is talking away to himself now, trying to get himself psyched up. Much more positive signs than we had seen from the world number one in recent games.

The movement still isn't right, but there are perhaps the first signs that his discomfort is easing as he gets stuck into Alexander Zverev's serve. But the German holds from 40-30.

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Still big problems here for Carlos Alcaraz, who is having to pick his battles wisely. A stark contrast to the endless sprinting about he was doing for the first two-and-a-half hours or so.

A missed backhand leaves him 0-30 down on serve but he continues to battle away, reaching game point and then holding from deuce.

A couple of aces help Alexander Zverev to an opening love hold in the fourth.

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Carlos Alcaraz is receiving treatment on both legs now, suggesting it is cramp that he's struggling with.

The focus remains on the initial problem area, that inside of his upper right leg.

Having been within two points of securing a place in the final, Alcaraz must reset in a fourth set.

Carlos Alcaraz saves the first set point with a measured drop shot, but Alexander Zverev seals the set with an ace.

Honestly, who knows where this goes from here.

Long from Carlos Alcaraz.

Four set points.

Carlos Alcaraz looks to have control of the point but narrowly misses the line and heads to the change of ends 4-2 down.

Alexander Zverev then makes a drop shot which Alcaraz does not bother to chase down, leaving the German a couple of points away from forcing a fourth set.

A huge serve down the middle from Alexander Zverev ensures the German returns serving duties to Carlos Alcaraz 2-1 up - and he'll resume with a 3-2 advantage after Alcaraz hits wide.

Carlos Alcaraz carefully watches an overhead to get up and running in the tie-break. His leg issue doesn't appear to have worsened, but his movement still isn't great.

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Alexander Zverev finds a couple of first serves to reach 30-30, before Carlos Alcaraz overhits an attempted lob.

In goes a closing backhand and we're going to another tie-break!

Carlos Alcaraz, still unable to move freely, slams a forehand down the line for 0-30.

Two more points needed.

More treatment for Carlos Alcaraz, once again just a game away from a first appearance in the Melbourne final.

Alexander Zverev, again serving to stay in this match, double-faults on the opening point.

Image source, Reuters

Quite concerningly, Carlos Alcaraz doesn't even jump as he makes a second serve, but still Alexander Zverev can't capitalise as the Spaniard moves to 30-0.

Ridiculous. Alcaraz swings a forehand into the corner to move to game points, and then makes a backhand winner into the other side on the next point to guarantee himself a tie-break.

I'm really not sure how he's doing this, but he is.

Carlos Alcaraz knocks a return into the net as Alexander Zverev levels at five games apiece.

The crowd rally behind Alcaraz. How much longer can he continue like this?

Carlos Alcaraz is barely moving.

Yet he gets to 30-30 and just two points away from the final.

Incredible viewing.

Back on his feet, Carlos Alcaraz returns to the court aware he is one break of serve from victory.

And it may very well be that is his best hope of reaching the final, if this is a serious issue.

The Spaniard does not look in a good way as he makes a couple of unforced errors.

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Carlos Alcaraz receives treatment on the inside of his upper right leg during a medical timeout, while Alexander Zverev is fuming that the Spaniard is getting longer than he should for what he perceives to be just cramp.

You can understand it given what is on the line here, but it's not exactly very sportsmanlike...

Carlos Alcaraz wins the next point as he rolls in a forehand, but there are concerned looks across the faces of his team members as they wait to see how serious this is for their player.

With essentially one full functioning leg for the time being, Alcaraz somehow manages to hold serve.

He's just one game away from a first Australian Open final, and he'll be desperate to secure that at the earliest opportunity.