Katerina Siniakova hammers a forehand into the back of Naomi Osaka's court and makes the line for 40-30. A huge shot on a high-pressure point.

The Czech holds to stay alive here.

Katerina Siniakova loses her focus as she whacks a forehand wide, after a member of the crowd disrupts her during her ball toss. That must be the third or fourth time the chair umpire has had to have a word now and it must be really frustrating for the players.

There's trouble for the Czech here at 0-30.

Harriet Dart looks deep in thought during the change of ends as she prepares to fight to stay in the first set.

There's a glimmer of hope at 30-30 after a double fault, but Marie Bouzkova holds her nerve, sealing the set as Dart puts a backhand into the net.

Katerina Siniakova let out her frustration after conceding the early break. She knows what a huge opportunity she has given herself, so immediately going behind is a real blow.

Naomi Osaka looks to the sky with relief after coming out on top in an enthralling point, landing a volley as she comes through from 15-30 down to hold serve for a 3-0 lead.

Harriet Dart is facing some pressure here as Marie Bouzkova takes advantage of some loose serving.

The Czech 25th seed opens up two break points and seals the game when Dart serves her second double fault of the game.

That was disappointing from Dart and Bouzkova will now serve for the first set.

Image source, Getty Images

Naomi Osaka gets fortunate with a bounce off the net cord which leaves Katerina Siniakova scrambling forward.

Osaka shows no mercy, lobbing her opponent to clinch the break, before offering an apology.

After suffering a massive setback in losing the second set, is that the bit of good fortune the 13th seed needs to press on to victory?

Katerina Siniakova faces an important fifth point at 30-30 in her opening service game.

The Czech nets, and Naomi Osaka will have the chance to make the perfect start to the deciding set.

Harriet Dart is showing signs of threatening Marie Bouzkova's serve with a couple of glorious winners.

The Czech 25th seed does come through in the end to hold as Dart skews a backhand wide, but that was more promising from the Briton.

Image source, Getty Images

Naomi Osaka must be wondering how she is in a deciding set here.

But she is, and she makes a positive start by holding serve from 40-30 to avoid conceding early ground to an opponent who is clearly growing in confidence.

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.

Image source, Getty Images

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.