Sabalenka falters with the first set in reach.

How significant a moment could that prove to be? A distraction from the crowd perhaps disrupted her rhythm - and Gauff has taken full advantage.

No shortage of twists already nine games into this one!

Jonathan Jurejko
BBC Sport in Melbourne

Sabalenka is stalled by a shout of support in her favour - and by the elongated shushes that follow.

Is that a smile from the Belarusian?

Not sure from this distance but certainly looked like it and would indicate how relaxed she is feeling.

Ah, and then she tenses up to hand the break back. Commentator's curse, etc, etc.

That's a promising start for Coco Gauff, producing a backhand winner to steal the first point off Aryna Sabalankea's serve.

Sabalenka composes herself before serving after a noise in the crowd but misses a backhand down the line to bring Gauff to 30-30.

Oh no! A double fault gives Gauff a break point and a chance to stay alive in the first set.

Jonathan Jurejko
BBC Sport in Melbourne

Coco Gauff lands a trio of confidence-boosting first serves to start the eighth game.

But when she does miss a first serve, a collective intake of breath from the fans follows.

Ignoring that must take extraordinary mental strength.

That she does though and manages to prolong the set.

Pat Cash
Former Wimbledon champion on BBC Radio 5 Sports Extra and BBC Sounds

Image source, Getty Images

Coco Gauff does come forward very early with her body weight. If she doesn't get the ball toss right, it ends up sort of behind her head.

But Coco Gauff does force Aryna Sabalenka to try and see this out.

Now, can she make the most of her final shot at the second seed's serve?

Russell Fuller
Tennis correspondent on BBC Radio 5 Sports Extra and BBC Sounds

There's so much power behind Aryna Sabalenka's shots. It's breath-taking to watch her in full flow.

Big cheers greet a stunning backhand winner from Coco Gauff. The crowd are desperate for the American to regain a foothold here.

Aryna Sabalenka fires back with a blistering forehand, before another bobbles off the top of the net and in for 30-30. More pressure.

Too powerful. Too accurate. Too good from Aryna Sabalenka so far.

Another commanding service game from the Belarusian is polished off with an ace to leave her a game away from the first set.

Lots for Coco Gauff to be thinking about here - but she came back at the US Open and she knows she can do so again here.

Russell Fuller
Tennis correspondent on BBC Radio 5 Sports Extra and BBC Sounds

The world number two position will be determined by who wins this semi-final tonight.

Aryna Sabalenka is currently number two but Coco Gauff would move from number four if she's able to win this match.

It's all about reaching a Grand Slam final tonight though.

Jonathan Jurejko
BBC Sport in Melbourne

Concerned groans from the Melbourne crowd as Coco Gauff double faults again.

That's five now.

Encouraging applause follows as the fans try to rally the American teenager, who is shading the support on Rod Laver Arena.

A booming forehand from Aryna Sabalenka clinches the break! She's hitting big and clean so far.

Huge disappointment for Gauff, with two costly double faults in that game. She can't afford to give Sabalenka cheap points when she's in this form.

Coco Gauff misses a forehand on her opening point but the big-hitting Aryna Sabalenka goes too big on a couple of aggressive returns.

A fourth double fault for Coco Gauff gives the defending champion 30-30, and yet another brings Sabalenka to deuce.

Can Sabalenka take advantage?

Pat Cash
Former Wimbledon champion on BBC Radio 5 Sports Extra and BBC Sounds

Aryna Sabalenka does really well at brushing double faults off. I think because her serve has been so up and down in the early parts of her career, she got to the stage where she had to accept that she'll serve some double faults and move on.

Image source, Getty Images

Gauff moves in to meet a second serve but she can't read it and nets for 40-0 before Sabalenka slams a forehand cross-court to wrap up a hold to love.

The Belarusian has either held to love or been broken on serve so far. No in-between.

Coco Gauff needs to start firing on serve quickly. After great work to respond in the previous game, she double-faults for a third time for 15-15.

But there are no signs of panic. A terrific lob beats Aryna Sabalenka as Gauff works her way to a comfortable first hold of serve to level the first set.

Jonathan Jurejko
BBC Sport in Melbourne

Nerves - and who handles them better - are likely to play a decisive part in this semi-final.

Both players have had issues with overcoming them in important moments over recent seasons.

Signs of tension from each player in the opening three games - particularly Gauff.

Strap in, folks. Could be wild.

Pat Cash
Former Wimbledon champion on BBC Radio 5 Sports Extra and BBC Sounds

It is an intriguing match-up. Aryna Sabalenka hasn't been challenged at all, which is always a concern for me.

If Coco Gauff gets going and plays well, we know she's lightning fast around the court.

That forehand was all over the place in the last round though. It wasn't consistent at all.

Image source, Getty Images

If you weren't already excited enough for this one, you should be now!

Coco Gauff is right back in this first set, defending superbly before moving in to the net to angle a volley out of reach and take her second break point.

Here comes Coco Gauff!

The American puts away a backhand winner as she launches an immediate response, shaking off a tight start to set up three break points.