Image source, Getty Images

Two big serves from Coco Gauff pull her out of trouble.

But a wicked forehand from Varvara Gracheva brings us back to deuce.

That's more like it. Coco Gauff pumps her fist after coming to the net and slamming home a return, fizzing the ball past Varvara Gracheva to save the break point.

But she's got another as Gauff finds the fence.

Jan Choinski is going all or nothing with some of these returns and they're not paying off right now.

But the Briton fights back with a mini break, whipping a forehand down the line for 3-3.

There's that serve we know Coco Gauff has up her sleeve.

An ace gets her service game up and running, but two unforced errors pull Varvara Gracheva level at 30-30.

Another one gives the world number 66 a first break point.

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Not the best way for Coco Gauff to start her title defence.

Four errors from the American hands Varvara Gracheva an easy hold.

*denotes next server.

Roberto Carballes Baena holds to love and we go to a tie break in this fourth set.

Naomi Broady
British player on BBC Radio 5 Sports Extra and BBC Sounds

They've got to enjoy the suffering as athletes.

Jan Choinski is going to be hoping he can take this into a fifth set.

Roberto Carballes Baena isn't finding it easy, he has done a lot of running and he is going to be feeling it.

Coco Gauff is about to start her US Open title defence.

The American third seed defeated Aryna Sabalenka in last year's final to win her maiden Grand Slam - aged just 19.

She faces France's Varvara, who is currently ranked 66th in the world.

The pair are warming up on Arthur Ashe.

Image source, Getty Images

Roberto Carballes Baena plods to the back of the court, wipes himself down with a towel and catches his breath for a few seconds with his hands on his knees.

It looks like the Spaniard's relentless scurrying around the court is starting to catch up with him in the heat.

Just as a I write this, he shows more gritty defensive work to force deuce, but Jan Choinski holds with a big first serve.

Naomi Broady
British player on BBC Radio 5 Sports Extra and BBC Sounds

Roberto Carballes Baena looks to be the one struggling now with the heat and the fatigue.

Image source, Getty Images

With the focus on Dominic Thiem's farewell and Jan Choinski in the past few minutes, let's catch up with some of the latest scores and results from around the courts:

• Alexander Zverev has wrapped up a four-set win over fellow German Maximilan Marterer.
• Former French Open finalist Casper Ruud is into a two-set lead against Bu Yunchaokete of China.
• Sixth seed Andrey Rublev is a set up against Thiago Seyboth Wild.

Coming up shortly... Coco Gauff begins the defence of her women's singles title against Varvara Gracheva on Arthur Ashe Stadium.

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

You would probably give Jan Choinski slightly greater than a 50% chance of taking it to a fifth set, given the way he is playing now.

A key moment at 30-30 but a backhand from deep in the court gets Jan Choinski out of trouble.

Roberto Carballes Baena then can't get a big first serve back in court and the Briton holds.

Both players make use of the ice towels during the break as temperatures edge near to 30 degrees Celsius.

After his farewell speech, Dominic Thiem receives a framed collage of his best moments at the US Open. Slap bang in the middle of it is a picture of him lifting the trophy aloft in 2020.

Thanks for the memories, Dominic.

Roberto Carballes Baena double faults to gift Jan Choinski a 0-30 lead.

The Spaniard then spoons a wild forehand wide to hand two break points to the Briton.

Big moment for Choinski this, but Carballes Baena recovers, coming out on top of a rally before finding some big first serves to hold.

Image source, Reuters

2020 US Open winner Dominic Thiem, speaking on court after his defeat by Ben Shelton in his final Grand Slam match:

"First of all, thank you so much for all of your support over the years. It's been 10 years since I played here for the first time.

"It's actually a really important moment for me because I had my greatest success of my career here on this court, in a weird 2020 in very strange and different circumstances. Unfortunately I had this success without any of you, so that was an amazing moment but on the other hand pretty sad.

"I'm super happy that I got the chance to play my last US Open, my last match here on this court and I can spend some time with you guys just to say thank you to all of you and to make the time up from what we missed four years ago.

"I'm grateful to everyone who came and everybody who got me the chance to play here for one last time."

There's times you think Jan Choinski has won the point because Roberto Carballes Baena is so far back on the court you can't see him on the television cameras, but then a return fizzes back at the Briton out of nowhere.

Choinski has to dig deep, saving three break points, to hold and keep his nose in front during this fourth set.

Image source, Getty Images

It looks like Dominic Thiem's Grand Slam career will be coming to an end fairly soon.

Ben Shelton, in a shocking pink, pale blue and white vest top, is two sets and 4-2 up against the Austrian.