Aryna Sabalenka is clearly frustrated and has taken a bathroom break.

Meanwhile, Madison Keys sits frozen, looking so calm and focused.

Contrasting emotions from the two semi-finalists.

Image source, Getty Images

And there we have it. Madison Keys has just wrapped up a first-set bagel in 30 minutes against the second seed.

Aryna Sabalenka mutters away to herself. What has gone so wrong for the Belarusian?

It's worth remembering Aryna Sabalenka has not dropped a set all tournament.

Aryna Sabalenka is known for her power, but it's Madison Keys impressing in that aspect at the moment.

That, and pin-point accuracy. The 17th seed is hammering off forehands and finding the corner every time.

She races to a 40-0 lead and has three break points.

Sabalenka mishits a backhand and it goes wide. Madison Keys will serve for a first-set bagel.

Madison Keys wriggles her way to a hold as Aryna Sabalenka gets slightly unlucky with a net cord.

Image source, Getty Images

Advantage Madison Keys and Aryna Sabalenka is furious with herself, directing her anger to her box.

However, she manages to pull the American back to deuce twice.

The first comes and goes as the second seed misses the line by mere millimetres.

The second falls limply into the net. Deuce.

What do we have here? Aryna Sabalenka has two break points of her own.

Well, well, well. I don't think anyone was expecting this.

Madison Keys is a double break up in the opening set against second seed Aryna Sabalenka.

Another double fault from the Belarussian and it's advantage and break point Keys. A long forehand means Sabalenka finds herself 3-0 down.

Image source, Getty Images

Aryna Sabalenka just hasn't hit the ground running here.

She puts away a backhand winner and an ace before letting Madison Keys claw the next few points back.

Some terrific play from Madison Keys at the net before whipping a backhand to the baseline and out of Sabalenka's reach. Deuce.

So let's see what Aryna Sabalenka responds with here. Perhaps there's some butterflies in the second seed's stomach.

Madison Keys, the underdog going into this match, has shown she shouldn't be underestimated.

Her forehand is working wonders at the moment and she gets the hold in her first service game as Aryna Sabalenka finds the net.

Image source, Getty Images

She can. Aryna Sabalenka double faults and hands the American a very early break.

That's not the start the upcoming new world number one will have wanted.

A third break point arrives for Madison Keys as Aryna Sabalenka fires a forehand long.

Can she get it this time?

Aryna Sabalenka saves the first with an ace before Madison Keys pushes a return into the net.

Deuce.

Wow. Three forehand winners from Madison Keys and she has two break points.

What a start for the 17th seed.

We're underway again on Arthur Ashe. Aryna Sabalenka serves first to get the second semi-final started.

*denotes next server

Jeff Tarango
Former American tennis player on BBC Radio 5 Sports Extra

It's Aryna Sabalenka's time. She just went into number one in the world. She's been working hard and been really diligent about improving her serve.

She won the Australian this year so she got the monkey off her back in that regard and she's just all forward, right now.

How can someone defend against that power.

Aryna Sabalenka knows she will take over as the new world number one on Monday, whatever happens tonight - or on Saturday.

But can she crown that moment by winning her second Grand Slam of the year?

She's the favourite, but Keys will have a strong backing from a crowd still pumped up following Coco Gauff's victory...

Jeff Tarango
Former American tennis player on BBC Radio 5 Sports Extra

Coco Gauff certainly does feel like she belongs out there.

It is very beneficial that Muchova made that such a challenge. she had to win it on the sixth match point because that got her through all the nerves and jitters possible when facing winning a match at this stage.

She is going to feel so much better in the final because of how hard she had to work to win this match. You can't buy that.

Next up on Arthur Ashe Stadium, we'll discover who Coco Gauff will face in Saturday's final.

Will it be Australian Open champion Aryna Sabalenka, set to become world number one next week?

Or will Madison Keys set up an all-American showdown that would certainly create a raucous atmosphere this weekend?

They are just heading out on to the court now!