Iga Swiatek has the mini-break but she'll feel it should be a 3-0 in the breaker.

A loose forehand gifts Marie Bouzkova a point on serve.

We're going to a tie-break!

Iga Swiatek converts the second break point with Marie Bouzkova unable to withstand the onslaught and hitting the ball tamely into the net.

On we go...

Image source, Reuters

Iga Swiatek is not giving this set up without a fight.

Some crunching groundstrokes take it to 15-30 before a blistering forehand return down the line brings up two break points...

Well, it's certainly not proving a straightforward day for the favourites in New York.

Flavio Cobolli, the fifth seed and runner-up at this year's French Open, had to battle hard to win a first-set tie-break but has lost the second set to Belgian 28th seed Alexander Blockx.

It might not be any surprise for Cobolli, who has already had to come from two sets down to win his opener, and a set behind in his second round match.

Image source, Reuters

Marie Bouzkova breaks at the crucial moment!

Iga Swiatek was putting everything into those break points and saves the first two.

But Bouzkova reads a whipped forehand on the third, replies with a superb forehand cross-court and a stretching Swiatek can only put the ball into the net.

Bouzkova will serve for the set...

Big opportunity for Marie Bouzkova as she brings up three break points.

Iga Swiatek saves the first but convert one of the next two and she'll be serving for the set.

Excellent hold from Marie Bouzkova.

She gives Iga Swiatek a brief opening with a double fault to start the game but quickly gets back on track.

The Czech does not give her opponents easy points, stays in the rally and is ready to pounce on errors. It all helps her hold with a minimum of fuss.

Pam Shriver
Five-time Wimbledon doubles champion on BBC Radio 5 Live

Fritz has played so much tennis the last eight or 10 years, the last six or seven inside the top finalists.

There's a lot of stress on US men when you think about it, since 2003 a US male hasn't won a US major. When you carry that kind of stress underneath, it can often lead to physical problems as well.

Wow!

We're heading to a fifth set on Arthur Ashe Stadium.

Francisco Cerundolo is eyeing a big shock - and a quite remarkable comeback - after recovering from two sets down against American ninth seed Taylor Fritz.

Frritz appeared in complete control, in a quarter of the draw which has really opened up for him, but now he's going to have to reset after losing the fourth set with a single loss of serve.

With more than three hours played, can Fritz find another gear after that huge setback?

Comfortable hold for Iga Swiatek.

The groundstrokes are looking good with a cross-court forehand winner taking her to 40-0.

Marie Bouzkova unleashes a fine return down the line to prevent the love hold but then fires into the net to leave Swiatek a game away from taking this first set.

New balls for Marie Bouzkova to work with and she uses them well, claiming her simplest hold of the match so far.

She holds to love, in fact, as the prospect of a first set tie-break creeps into view.

We're starting to see the quality of Iga Swiatek shine through.

She's still not having it all her own way but the power is there and she is picking her moments to move in nicely.

A hold to 30 keeps it on serve in the first set.

Marie Bouzkova has her first hold of serve as well.

The Czech 25th seed is taken to 40-30 before Iga Swiatek pushes a forehand long.

Signs of frustration building for Taylor Fritz.

It's a scrappy contest alright. But, on the 18th break point of the match, Francisco Cerundolo makes the break through in the fourth set!

Both players have surpassed 40 unforced errors, which tells you all you need to know about the type of contest it is.

But are we on for a remarkable comeback here?

There's still plenty of work for Argentine 24th seed Cerundolo to do, but he has all the momentum now as he seeks to shock the American ninth seed from two sets down.

A hold at last!

After four long games, in which the server has been made to work so hard - and, ultimately, come up just short - Iga Swiatek breezes through this one.

Still on serve but that should help settle the six-time Grand Slam champion down.

Image source, Reuters

Four breaks on the trot and we're back on serve.

Iga Swiatek is hammering away from the baseline and gradually wears down the stout defence of Marie Bouzkova.

A thumping backhand winner brings up break point and when Bouzkova goes long, the Pole is level once more.

Well, three games in and we've yet to see a hold of serve.

Iga Swiatek came in as a clear favourite but Marie Bouzkova has shown more than enough already to make it clear that the eighth seed is not going to have it all her own way.

No shortage of unforced errors early on but it is Bouzkova with the advantage once more in this first set.

Image source, Reuters

Italy's men's fifth seed Flavio Cobolli has edged a tight opening set against Belgian Alexander Blockx, eventually taking it in the tie-break 7-4.

Women's fifth seed Mirra Andreeva also won the first set against opponent Nikola Bartunkova - but the Russian has a fight on her hands in the second set.

Bartunkova lost serve in her opening game but broke twice to take a 4-2 lead, however Andreeva has just responded to bring a topsy-turvy set back on serve.

Immediate response from Iga Swiatek.

She shows her power with a flurry of aggressive groundstrokes to move up 15-30 but Marie Bouzkova looks to have done enough as she gets to 40-30.

However, a double fault takes it to deuce and, while two break points go begging, Swiatek converts the third with a volley at the net.