Tournament officials are on their radios, trying to work out exactly what is going on up in the stands, with whoever has caused the disruption still yet to be removed.

The rest of the crowd are making clear their feelings. 'Kick them out' they chant.

Karolina Muchova is having a medical timeout as Coco Gauff runs through her serve.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Play has been paused by some fans shouting out from the top tier, seemingly in some kind of vocal protest.

But the pair are so high up in the stand it is impossible to know what their cause is.

Security are nowhere to be seen.

Now then, there's something going on in the crowd.

It sounds as though a group of people are purposefully disrupting the action in some form of protest.

The match has been paused for the time being, with both players taking a seat.

The start Coco Gauff both wanted and needed.

Karolina Muchova is able to stay in touch with a forehand winner for 40-30, but she's unable to apply any further pressure.

An opening hold of serve to begin set two for the American.

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

Karolina Muchova is a comeback player.

She moves gracefully, she absorbs the pace well and yet she comes up with shots you don’t see very often– guile should be her middle name.

It’s really entertaining to watch.

Jonathan Jurejko
BBC Sport at Flushing Meadows

"I've gotta feeling, that tonight's gonna be a good, good night..."

The DJ on Ashe is feeling confident.

Black Eyed Peas blasts out as we wait for Muchova to come back.

Right then, what's in store for us in set two?

Will that final game have settled Coco Gauff back into her rhythm, or can Karolina Muchova use her fightback to assert herself much sooner in this set...

Image source, Getty Images

There has been sweating, dizziness and even the warning from Daniil Medvedev that a player could "die" in the hot conditions at the US Open.

Players once again have been struggling with the heat on one of tennis' biggest stages in temperatures around 35C on Thursday.

With the heat turned up at the sharp end of the US Open - quite literally - the question is, how hot is too hot to play tennis?

For the tennis players out there - or any sporty folk for that matter - what tips do you have for keeping cool in such conditions?

And indeed, for simply following the action during the UK's own heatwave? Out of necessity I've got the windows wide open, despite the cast of A Bugs Life attempting to make its way indoors.

Let us know using #bbctennis.

BBC Sport has looked at the impact on athletes bodies and what organisers do to protect them. You can have a read of that here.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Muchova slaps her thigh in frustration when she knocks that forehand into the net for set point.

Her head bows when she hands it over. Within seconds she goes off court.

David Law
Commentator on BBC Radio 5 Sports Extra

I dare say Gauff is pretty relieved because that set was going swimmingly, and then it was going nervy.

Image source, Getty Images

What a bizarre conclusion to a topsy-turvy the first set.

After doing so well to get back into contention from two breaks down, Karolina Muchova sees the first set disappear with a break to love.

Relief for Gauff - and the New York crowd. That got a bit nervy towards the end.

Ah, the pressure is on again for Karolina Muchova, who looks frustrated by a poor miss at the net.

It could prove costly too, at 0-30.

Another miss hands over three set points!

Jonathan Jurejko
BBC Sport at Flushing Meadows

Muchova's earlier troubles were illustrated by a whopping 13 unforced errors and just two winners.

But she has settled and now it is Gauff who looks edgier.

Muchova getting more balls back into play and asking more questions.

David Law
Commentator on BBC Radio 5 Sports Extra

Karolina Muchova is a different player from three games ago – and Coco Gauff is rattled.

She is trying to hold it together, because that is a real shock to the system.

Wow!!

Who saw that coming just three games ago?

Coco Gauff was storming to the opening set at 5-1 and two breaks of serve to the good.

But she has faltered, and Karolina Muchova has taken full advantage. A missed forehand from the American brings us back on serve. Remarkable!

Huge game for Coco Gauff here.

Karolina Muchova is growing in confidence with every miss the American makes - but now the 10th seed is starting to make her shots count too.

She closes in to put away a volley, setting up two break points!

Jonathan Jurejko
BBC Sport at Flushing Meadows

Spotted a guy outside Ashe wearing Stars and Stripes from head to toe.

Bandana, T-shirt, board shorts and socks.

"I'm rooting for a Muchova v Sabalenka final," he laughed.

David Law
Commentator on BBC Radio 5 Sports Extra

Under pressure, that Coco Gauff forehand doesn’t fill you with confidence.

Karolina Muchova is just starting to get a foothold.

Things are starting to turn here.

Image source, Getty Images

Groans in the stands as another unforced error puts Muchova on course for another hold.

This is definitely a dip, rather than a blip, for Gauff.

She's lost eight of the last 10 points, but here's a second chance to serve out the set. It's all gone a bit flat inside Arthur Ashe Stadium.

Nerves? Tension? Just a blip in an otherwise faultless opening set?

Coco Gauff knows she still has a buffer here, but would probably like to get the set wrapped up on Muchova's serve, rather than allow the Czech another look at hers...