That's better from Harriet Dart as she shows fight from the back of the court and forces Marie Bouzkova into a mistimed backhand.

Dart goes on to seal the game by punishing a looping return by stepping inside the court and putting a backhand winner away.

This looked like it was close to a foregone conclusion when Naomi Osaka led by a set and a break with one hour on the clock.

But Katerina Siniakova is still fighting half an hour later - and now will believe she has every chance of causing a big upset on Arthur Ashe Stadium with Naomi Osaka struggling to rediscover her best level.

Osaka has left the court before the deciding set.

Mark Woodforde
17-time Grand Slam doubles champion on BBC Radio 5 Sports Extra 2

Katerina Siniakova might have just loosened up! A couple of points fell her way and momentum took on a bigger proportion for her.

Naomi Osaka seems to be carrying a bit of baggage. She is not out of it, but it's a set apiece.

Harriet Dart's forehand wasn't on point during that last game and she practices it during the change of ends.

Marie Bouzkova sets about consolidating her break with some strong first serves to get to 40-15, before polishing the game off with a lovely forehand winner.

The Czech 25th seed is in control at the moment.

A wonderful backhand winner down the line from Katerina Siniakova puts her back on track.

Two set points are coming her way after a faltering Naomi Osaka nets, but the 13th seed unleashes a forehand winner to stay in touch.

No bother! A final winner from Siniakova clinches the set with a fourth straight game from 5-3 down. Incredible.

Katerina Siniakova tilts her head to the sky after beginning her bid to level the contest with a double fault.

She has done well to reduce those in this second set, but will hope that doesn't prove costly here.

Abigail Johnson
Commentator on BBC Radio 5 Sports Extra 2

That is the biggest unforced error of the day from Naomi Osaka.

Naomi Osaka looks stunned!

From serving for the match, the two-time winner now faces losing the second set after an excellent return from Katerina Siniakova seals a third successive break of serve for the Czech.

What a remarkable momentum swing on Arthur Ashe Stadium.

Image source, Getty Images

Harriet Dart double faults to put herself in a spot of bother at 30-30 before planting a forehand into the net to offer Marie Bouzkova the first break point of the match.

Bouzkova takes it as Dart then drags a forehand wide.

The Briton was made to pay there for a couple of unforced errors.

How quickly things can change.

From fighting to stay in the match, Katerina Siniakova has break points to set herself up for the chance to serve out the set!

Naomi Osaka rediscovers her first serve in time to reach deuce, but Siniakova will have another look...

Katie Smith
Presenter on BBC Radio 5 Sports Extra 2

We get a closer look as we see Naomi Osaka in her outfit this time out.

Still basketball-inspired and a looser jersey type top with a flared skirt.

There's plenty of fight still in Katerina Siniakova!

The doubles specialist digs in to hold from 30-30 and level the second set from 5-3 down.

Image source, Getty Images

Naomi Osaka hasn't given much away out on court today, but a quick glance to her team after her latest miss suggests she's just starting to get a bit frustrated here.

But, from 30-0, Siniakova makes a couple of misses to bring Osaka into contention and two points from victory.

Harriet Dart is edging the rallies so far but she can do nothing about that as Marie Bouzkova nails a perfect drop shot.

The Czech 25th seed seals the game as Dart is unable to get a forehand over the net.

Naomi Osaka concedes the game as she nets a forehand, and a first chance to book her place in the third round slips from her grasp.

Four consecutive breaks of serve, and it's very difficult to call which way this is going to go.

Image source, Getty Images

Goodness me this is a scrappy end to this second set.

For a fourth straight game we have break points, and two chances for Katerina Siniakova to deny Naomi Osaka as she serves for the match.

Image source, Reuters

It's a confident start from Harriet Dart who has settled into her stride as she wins a couple of rallies from the baseline.

Dart seals the game as she steps into the net with Marie Bouzkova unable to find a way past.

Abigail Johnson
Commentator on BBC Radio 5 Sports Extra 2

Katerina Siniakova has been scrambling, and she has not been able to put all her colours out on court today against the former world number one, Naomi Osaka.

Just when it looked as if Katerina Siniakova had gained a foothold, Naomi Osaka fires back to reach three break points.

Can the Czech go again here?

Siniakova sees off one break point but concedes the second. That will be such a huge disappointment after waiting so long to finally make a breakthrough of her own.

Osaka will serve for a place in the third round.

Image source, Getty Images

Harriet Dart is due on Court 11 shortly against Czech 25th seed Marie Bouzkova.

Dart is one of only two remaining Britons left in the singles at the US Open, alongside Fran Jones, who plays later today.

Dart and Bouzkova have only met once before, with the Briton winning in three sets at Eastbourne in 2024.