The more Sabalenka gets irritated, the more the home crowd will get behind Pegula and on the back of the top seed.

But the scream of frustration is from Pegula this time, as she drops an easy shot into the net.

Better from Sabalenka, who holds to 15.

Can Pegula hold serve? Yes She Can.

She holds to 15, with Sabalenka visibly annoyed as she continues to overhit many of her ground strokes - before chucking her racquet to the ground as Pegula confirms the hold.

Sabalenka's body language is not inspiring confidence at this stage.

Huge cheers from the crowd as former First Lady Michelle Obama is shown on the big screen.

She will hope to watch fellow American Jess Pegula serve to stay in this first set.

Image source, Getty Images

Annabel Croft
Former British number one on BBC Radio 5 Sports Extra 2

That is two points in the game where Sabalenka should have won the point. She was in the driving seat but she just over-cooked that forehand. She looks so disappointed.

Sabalenka is behaving like this is a terrible set and yet the standard has been so incredibly high. If she is not careful these emotions are going to get the better of her.

What a time for Aryna Sabalenka to throw in the first double fault of the match!

Advantage Pegula for the first time, her first break point, and a huge cheer from the crowd... until she is forced wide by another booming serve and Sabalenka saves the break into the open court.

Sabalenka moves to advantage with an overhead winner at the net, and sees off the break threat with another unreturnable serve.

Aryna Sabalenka clearly outguns Pegula in terms of power - four aces, 11 winners and a good record on second serves.

But whenever she is taken to a rally, Sabalenka is being outplayed by Pegula. Case in point here as Sabalenka puts one in the net, it's 30-30 on the top seed's serve and the American crowd is roused.

Sabalenka responds with another 117mph serve, nothing anyone can reasonably do against that.

But then, after another rally, Sabalenka goes long. Deuce, and half a chance for the third seed...

Image source, Getty Images

This is the first time the top-four ranked players have reached the women's semi-finals at any Grand Slam since Wimbledon in 2009, and it hints at a shift away from the unpredictable landscape that has characterised the women's game in recent years.

Since the start of 2020, there have been 13 different Grand Slam women's singles champions - compared to just seven on the men's side.

But the big names are all here, so after top seed and two-time defending champion Aryna Sabalenka faces Jessica Pegula, Elena Rybakina - who will replace Sabalenka as the new world number one on Monday - takes on fourth seed Coco Gauff afterwards.

Both women are getting more than 75% of their first serves in, and averaging over 100mph. We have not had a single double fault yet.

It is really helping add to the high quality we have seen so far in this match.

Pegula maintains that theme with a hold to 15.

Image source, Getty Images

Annabel Croft
Former British number one on BBC Radio 5 Sports Extra 2

There is a self-assured feeling with the way Pegula is playing this match at the moment. She looks very at ease and looks like she is playing within herself and yet, this is the highest level I have seen her play at.

She is just serving better than I have ever seen.

Aryna Sabalenka is hitting her serves with immense power tonight. She's up to four aces already, with an average speed of 105mph.

When she doesn't get the first serve in, Pegula is dealing well with her power on court. But Sabalenka does enough to hold to 30 again.

Image source, Getty Images

As she has all tournament, Aryna Sabalenka is wearing a chunky gold necklace with coloured jewels in it as part of her outfit for this game. It's very Infinity Gauntlet coded, for any other Marvel nerds in the audience.

Anyway, Pegula has just held to love. Both women have been excellent on serve so far.

Image source, Getty Images

There are a few latecomers into the arena, and the umpire calls out one spectator for not taking their seat as Sabalenka is about to serve. Given the focus on fan behaviour at this tournament, that may be one to keep an eye on.

Back on court, Jess Pegula continues to strike the ball superbly - she is up to four forehand winners already.

But Sabalenka responds with a 117mph ace, before holding to 30.

Image source, Getty Images

Annabel Croft
Former British number one on BBC Radio 5 Sports Extra 2

Sabalenka is looking very sharp.

I have always thought she loves to go after the ball. She has often said that she knows what she can do in practise and she will just implement it in a match. When the going gets tough in a match, she will back herself.

But Pegula is hammering that ball. She is on it and is seeing the ball very clearly. Her team is looking very intense down there, but they must be happy with how she is playing.

This is a good level of tennis from both of them.

Jess Pegula has a little bit of strapping on her right thigh. That's new, but she doesn't seem to have any obvious issue with movement so far.

And she holds as Sabalenka goes long.

The first sniff of a break in this match comes from Aryna Sabalenka, a dismissive return winner puts her 0-30 up.

Jess Pegula responds perfectly with the first ace of the match, then Sabalenka is left frustrated as a return flies wide. 30-30.

But as Pegula finds the net, it is break point Sabalenka...

A roar from the crowd as the American saves it with a fine forehand out wide. Deuce.

Image source, Getty Images

Emily Salley
BBC Sport

"Oh, wow".

That was Coco Gauff's surprised reaction when she learned she was part of a US Open semi-final line-up the like of which has not been seen in more than half a century.

For the first time since 1975, the top four seeds in the women's singles draw have reached the semi-finals at the New York major.

Now, 51 years later, the stars have aligned once again with Aryna Sabalenka, Elena Rybakina, Jessica Pegula and Gauff all surviving three-set thrillers to create a semi-final line-up for the ages.

"How amazing - it doesn't get much better than that does it?" former British number one Annabel Croft said on BBC Radio 5 Live.

"It has felt fantastic the whole tournament - I just feel like I've been more enthused about looking forward to the match-ups this Grand Slam than any I can remember."

So far, this match is of the high quality we hoped.

Sabalenka goes a little wavy with her serve, missing her first effort on the opening three points. But she holds to 15, and we stay on serve into the first changeover.