Image source, Reuters

That is brilliant from Konta, she smashes a return straight across court on the front foot, it zips away, leaving Petkovic stranded, rooted to the spot. 

Konta has no trouble following that up for the game. She's now serving for the match!

Petkovic has definitely recovered from her wobble now. She's all about the front foot, smashing one straight at Konta, who can only return with a reflex action into the net.

Petkovic challenges another late decision - once again the line umpire is right, and Konta comes back in at 30-30. 

A tremendous back-hand return stops Petkovic in her stride and sets up another break point but Petkovic comes back strongly again for deuce.

How close do you want? Petkovic is convinced her forehand was in and challenges the call. There's a nano-meter in it, maybe two, but the decision is correct. 

That could have turned the game, from 40-0 to 30-15, but Konta sees it through. 

She's looking comfortable and so she should. Surely she can't be caught now? The last 16 is beckoning. Is another seed set to fall?

Excellent stuff from Petkovic to see off Konta's advantage, a bold front-foot push to the edges. 

And the German really is coming back into her own again as she flashes one to the far corner, no way Konta was reaching that. The Briton then shoots too long and Petkovic holds.

Konta may have disappeared off the court while Petkovic was getting treatment but she's made sure to leave nothing behind on her return. 

Here she is scampering around in search of a cross-court winner, which she finds, and it looks like she's got more running still as she sets up two break points.

Can Konta seal it with a flourish? This forehand attempt goes wild, off into the distance. And Petkovic puts everything she has into the next rally to save the second. Deuce.

It's clearly the heat that's affecting Petkovic. She's got what looks like a very chilly towel draped across her shoulders and is taking on lots of water. 

Only the posh French stuff. She looks to be improving though and now digs into a banana. 

Konta is here and we are set to get back under way. How long can Petkovic last?

There's a medical time out. Konta is heading off away from the court while Petkovic gets treatment.

Can Konta keep her focus? She's so close to a brilliant win. 

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Petkovic really is moving very slowly indeed. Is there going to be any kind of response from the German? 

Konta isn't bothered. She's totally focused on what's at hand - a place in the last 16. This one zooms by.

Konta's barely ruffled but Petkovic is now in conversation with her trainer, courtside. She looks like she's suffering. Her blood pressure is being checked. 

It's the heat you must think. Whatever it is it's clearly having an effect - Petkovic with a double fault and Konta's two to the good in this one, too. 

Konta senses something turning. She goes on the attack with a forehand winner, forcing the break point. Another sluggish Petkovic shot is called out by the line umpire but overruled. No challenge from Konta - she doesn't need it. Petkovic looks to be running out of steam. She's doubled over now.

The umpire's been on her phone. Petkovic's trainer has been summoned, seemingly. You can just about pick up the words - The German's not feeling well apparently.  

Konta picks up from where she left off, zooming into a 30-0 lead. She's dragging Petkovic all over the court - a strategy to tire her opponent? 

It's working if it is, as Petkovic slumps into a return from Konta's serve. 

Image source, Reuters

Piers Newbery
BBC Sport at Flushing Meadows

"It's a mark of her rapid improvement that Konta is playing well below her level against Muguruza in the previous match but still leads the 18th seed. 

"If she can "stay in the present", as has been her mantra in New York, this match is there for the taking."

And she only needs one! Petkovic hits a serve long - she looked like she'd given up on that. 

Jo Konta is just one set away from the US Open last 16.

Brilliant from Konta. Petkovic looked so assured early on but now she's making silly mistakes, Like this forehand smash into the net.

Four set points for Konta.

Konta hits back straight away. She lost the first two games of the match, and the first two points of the tie-break. But now she's woken up again (again).

Image source, AFP

Both players have dropped back now. It's all coming from the baseline and we're seeing some very long exchanges. 

Petkovic has got the stamina, and the eye for the moment to strike.

Yes indeed. Another slow second serve is punished by Petkovic. We're headed for a tie break. 

That winner would put a spring in anyone's step. Konta is no exception. Can she harness it? 

Petkovic has been here many times before. She's poised to strike, quick to earn a 30-15 lead. Konta then double faults. Two break points for Petkovic. 

Konta forces her into an error. One down. Petkovic then drifts one wide - she went for the jugular and missed. Surely not yet another break?