Gauff takes the hold to 15 with a couple of points won behind her second serve in there. That will give her a confidence boost.

Can she start to make inroads on the Osaka serve?

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

Coco Gauff's facial expression today is lighter.

It felt the other day she was playing with a dark cloud over her, she looked tormented.

She doesn't look like that today.

While Gauff struggles to find a bit of rhythm Osaka is totally locked in.

She puts another hold to love on the board.

Image source, Getty Images

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

Image source, Getty Images

I think Osaka is playing about as well as I've seen her play since she returned as a mum onto the tour.

There's a lot more security about the way she hitting the ball, she looks really dialled in mentally, she's moving well.

It's very reminiscent of the way she was winning those Grand Slams, the way she's hitting the ball.

When she's on song, she's beautiful to watch - she makes the game look so easy.

Osaka fires long and that's a sigh of relief for Gauff as she takes the hold having been taken to deuce.

Osaka still with that break advantage.

Image source, Getty Images

A big driving backhand from Osaka is followed by an impressive bit of movement which leaves Gauff only able to dump into the net for deuce.

Osaka looks locked in.

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

Gauff is netting more balls rather than hitting them long. She's actually almost turning the racquet face over and swinging a little bit early but it also looks like she's not releasing and having a lot of freedom with that [forehand] stroke.

It's weird how she's hitting way more balls in the net rather than long.

Image source, Getty Images

Ah. Gauff begins her service game with a double fault.

There's then a nervy moment as she misses her first serve on the next point but a rallying cry from the crowd helps her move ahead in the game.

Gauff wins her first point on the Osaka serve for 40-15 but a driving forehand gives Osaka the hold.

No trouble so far for Osaka.

A brilliant hold to love sprinkled with a couple of aces gets Gauff on the board in this match.

Those serving problems look to be in the rear view mirror for now.

Image source, Getty Images

Osaka follows up that break with a hold to love. A really impressive start by the two-time champion.

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

There's already tension in the match. You can feel it. It's incredible how you can already feel it in the crowd.

What a start by two-time champion Osaka.

Gauff slams into the net and there's the break of serve at the first opportunity.

Already we're seeing some fairly lengthy rallies but it's Osaka who finds herself with two breaks, despite Gauff hitting four first serves in her opening game.

Just an hour and four minutes, with seven aces and 21 winners - it was a pretty dominant display from Iga Swiatek.

But she's still not satisfied - she delays her on-court post-match interview to send a text to her coach asking him to book a practice court.

Image source, Getty Images

Jonathan Jurejko
BBC Sport senior tennis reporter at Flushing Meadows

Wondering why this hotly-anticipated match between two of the biggest stars on the WTA Tour is not in the New York night session?

It's because this weekend is the Labor Day holiday in the United States, meaning this second day-time match is actually viewed as the primetime slot.

Good news for those of you back in the UK wanting to watch/follow our coverage.

Away we go. Third seed Gauff gets us under way.

* denotes next server

Coco Gauff has been struggling with her serving. It's a problem that's plagued her all year and has followed her to Flushing Meadows.

Earlier in the tournament she switched her team up to try and tackle those problems and she broke down in tears after her second-round match having struggled with a significant number of double faults.

But the American looked confident and sharp as she beat Poland's Magdalena Frech in the third round.

Her return game is still looking strong while her movement is also sublime.

But Naomi Osaka has looked in fine tune in New York too so this should be fascinating.

Jonathan Jurejko
BBC Sport senior tennis reporter at Flushing Meadows

These two players have met here before - but the circumstances were very different.

In 2019, world number one Osaka was the US Open defending champion, while Gauff was a 15-year-old prodigy playing on Arthur Ashe Stadium for the first time.

A captivating evening saw Osaka win 6-3 6-0 as a teary Gauff became overwhelmed by the occasion - and afterwards consoled her opponent in heart-warming scenes.

Now, Osaka is the lower-ranked player after having a baby two years ago and Gauff ranked third in the world.

This feels like the first proper 'popcorn' match of this year's tournament.

Image source, Getty Images

Second up at Arthur Ashe Stadium is the eagerly awaited women's fourth-round tie between Naomi Osaka and Coco Gauff.

Two-time US Open winner Osaka overcame 15th seed Daria Kasatkina 6-0 4-6 6-3 in the third round, and at times looked like the player that won four Grand Slams between 2018 and 2021.

Gauff - who Osaka described as "like my little sister" - is looking to become the queen of New York for the second time in three years.

The 21-year-old world number three made light work of Magdalena Frech in the third round, beating the Pole 6-3 6-1 in one hour and 13 minutes.

Gauff edged the head-to-head with three wins to two, but whichever way this one goes, it promises to be a fascinating match-up.