Konta is not quite in the gear she was in during the early stages, letting her coaching team of Wim Fissette and Andrew Fitzpatrick know that as she exchanges a few words with them after holding to 15.

Jo Konta blows a golden chance to break at the first opportunity in the second set. Could it prove costly? Probably not too much.

A wonderful hooked forehand gives Konta advantage but Krunic wipes that out with a controlled stretch to guide past Konta at the net.

Konta has another chance to nick an early break, however, when Krunic takes her eye off the ball for a split-second and cracks an overhead into the net.

Oh. What happened here? Konta misreads the Serb's next serve as the ball wriggles through her defence. Baffling. Krunic seizes the fortune with some sound attacking shots to hold.

Image source, Reuters

David Law
Radio 5 live sports extra commentator at Flushing Meadows

There is a large, heavy black cloud above the court, hopefully it will not dump any rain on New York today.

Piers Newbery
BBC Sport at Flushing Meadows

Konta's Belgian-Anglo coaching team of Wim Fissette and Andrew Fitzpatrick are seated in the front row, poker faces firmly set, but they will have plenty to analyse after that first set. Team Konta are methodical in their preparation and it was a surprise to see her let such a big lead slip.

In fairness, it can't be easy to play elite level sport with your nostrils full of the sweet smell of doughnuts.

Much better from Jo Konta as she discovers her mojo to take the first set.

The Briton plants down an overhead which Krunic scampers over to but can't get back over the net. That's 30-0.

Krunic comes out on top in a wonderful rally for 30-15, but a deep backhand from Konta in the next point leaves the Serb stretching - and doing the splits - to no avail.

Konta wraps up the opener at the first chance. Job, eventually, done.

Image source, AFP

Naomi Broady
British number three on BBC Radio 5 live sports extra at Flushing Meadows

The US Open have ball men and women, as well as the kids. I think it is the only grand slam that does it.

Now it is the turn of Krunic to lose her focus again. More twists and turns in this opening set than a log flume.

Konta has three break points as she reasserts her control - she only needs one.

Another double fault from Krunic hands over the game on a plate. Konta didn't even need a knife and fork.

Image source, Reuters

Naomi Broady
British number three on BBC Radio 5 live sports extra at Flushing Meadows

I don't think Jo will be very unhappy with herself, she has made a lot of errors. Krunic does make you play, but these are unforced errors rather than forced errors.

A deep serve from Konta can only be blocked back by Krunic on break point number one, the Brit galloping forward to wallop a backhand winner.

But the seventh seed can't save the second break point for the Serb, who has - almost miraculously - battled back from 4-1 down with three games on the spin.

Image source, AFP

Looks like Naomi might have jinxed Jo... Krunic has two more break points...

Naomi Broady
British number three on BBC Radio 5 live sports extra at Flushing Meadows

Sometimes when you go 4-1 up you can lose your focus, especially when there is not much rhythm and fight from your opponent

I think it will just wake Jo back up and she will not get broken again.

Back-to-back games by Aleksandra Krunic for the first time in the match leaves Johanna Konta with some food for thought. The Serb, whose highest ranking was 62 in July 2015, is under pressure on her serve again, but alleviates that with an ace down the middle and goes on to seal the game.

Image source, EPA

The last time we saw Marin Cilic he was in tears on Centre Court during his Wimbledon final defeat by Roger Federer, as he was hampered by a badly blistered foot. The Croat hasn't played since because of an abductor injury but seems in decent fettle so far in New York - the 2014 champion leads American Tennys Sandgren by two sets to love but trails 3-1 in the third. Sandgren, you may remember, was due to be Andy Murray's opponent before the world number two pulled out on Saturday.

Jo Konta is not going to have everything her own way it seems. Aleksandra Krunic lets fly with a backhand which dusts the net to bring up another break point and, after that cannot be converted, takes her second chance to show that she can be competitive against the seventh seed.

Naomi Broady
British number three on BBC Radio 5 live sports extra at Flushing Meadows

It is quite breezy today, it could be contributing to why the pair have served a couple of double faults.

The best thing to do is bring your margins in and not hit so close to the lines.

Image source, AFP

This one might not take up much of your Monday evening. Johanna Konta rattles off her fourth game on the bounce as she piles the pressure on Aleksandra Krunic's serve.

The Serb produces another double fault for 40-30, looking over to her box as if to say 'what can I do?'

Konta smells blood, pinning her opponent back with her power and closes out the game as Krunic gets strings to the ball but can't return to safety.

Naomi Broady
British number three on BBC Radio 5 live sports extra at Flushing Meadows

From my experience of playing Krunic, her anticipation is incredible. When she reads you the right way, she is there before you make contact with the ball.

It is quite overcast today and I think Jo's game is more suited if it does heat up, while Krunic will enjoy the cooler conditions.

Click the tab at the top of the page to hear live radio commentary.

Johanna Konta starts strongly as she looks to hold after that previous break, moving 30-0 ahead on the way to what appears to be a comfortable hold.

Hang on... Konta loses focus momentarily, two double faults helping the 24-year-old Serb win the next three points. Break point...

Konta responds by boshing an ace out wide for deuce, then makes it a pair to seal a wobbly hold.

Aleksandra Krunic is all too aware of the threat posed on the opposite side of the net, knowing full well Jo Konta is keen to pounce on her serve at the earliest opportunity.

A couple of double faults from the Serb instantly pile on the pressure, Konta clinching her first break point of the match by drilling a stunner of a backhand winner down the line.

Image source, AFP