One match points goes as a stretching Krunic slaps a forehand into the net...

WOW! A stunning forehand winner makes it 40-0 and three match points...

A wonderful stretching drop-shot can't be returned by Konta and a fistpump from Krunic puts her 30-0 ahead...

Nerves? Pah! An ace down the middle puts Krunic 15-0 ahead...

Wonder what's going through Krunic's mind at the changeover...

Sean Cox: C'MON JO !

Konta shows guts to fight back from 30-0 down to 40-30 in her favour, kicking off the revival with a thumping ace, then sealing it with a sensational second serve that leaves Krunic reeling and unable to recover when a blistering backhand comes back from the Brit.

If the world number 78 is going to beat the seventh seed then she will have to do it on her own serve...

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

Jo doesn't look herself, especially in the third set, her body language is not what you normally see, her shoulders are down.

She will make Krunic win it, she will not give this away.

Krunic's team are getting besides themselves, it will be a huge win if she can break Jo and win the match.

Image source, Rex Features

Let's hope Naomi Broady's positivity finds it way to Jo Konta.

Nope... Konta leaves a serve at 15-15 thinking it is out - it isn't - and then goes on to lose the game quick-sharp.

That's three games on the bounce for the Serb and leaves her one more away from a memorable victory...

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

We were in the same situation at the start of the set and Jo broke straight back, so I am expecting that to happen again when she gets up from her chair.

This is getting serious now. Krunic goes a break up for a second time in this deciding set - and time is running out.

A backhand winner by Krunic is wiped out by a big first serve from Konta (we haven't said that often today) which allows the Brit to hammer a defensive lob into the ground and out of play for 15-15

But two more unforced errors from Konta gift a par of break points to the Serb. The first goes when a second serve kicks up and Krunic just gets under her backhand return.

Konta finds her first serve when she needs it, but still it isn't good enough as Krunic cracks a beauty of a backhand winner.

Image source, Rex Features

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

Krunic is serving really well now, she is often serving the slice up the T on the advantage side which moves away from Jo and she is really struggling with it.

She is such a fantastic player, she is anticipating Jo's shots and she is feeling she has to play better than she normally does to play winners.

Justin Stone: I want Jo to win but I'm not feeling it, she'll fight to the end though.

Konta edges 30-15 ahead and then gallops forward to dispatch a forehand volley with Krunic on toast at the back of the court. Only, Konta doesn't dispatch it safety. She wallops it long. That gives the Serb hope and she battles back to hold.

Anyone care to call this one? Me neither.

Nothing is easy for Johanna Konta out there on Grandstand court. You suspect if she went for a hot dog then she'd have to wait for them to stock up on finger rolls before she got her food. And then they'd probably run out of mustard, meaning she'd have to wait longer still.

A battle of a fifth game sees Konta again taken to deuce on her serve, the Brit stepping into a backhand down the line, then springing across to bat the return down the opposite flank.

That gives her advantage and when Krunic swipes a backhand long, Konta yelps in delight. That tells you how much she needed that.

Some big names (and big fellas) are into the second round in the men's singles. Wimbledon finalist Marin Cilic has beaten Tennys Sandgren 6-4 6-3 3-6 6-3, John Isner defeated Pierre-Hugues Herbert 6-1 6-3 4-6 6-3 while Jo-Wilfried Tsonga got past Marius Copil 6-3 6-3 6-4.

Image source, Reuters

Piers Newbery
BBC Sport at Flushing Meadows

A little over an hour ago, the Grandstand Court felt like a sleepy corner of Flushing Meadows as Konta rolled into a 4-1 lead. Fast forward 70 minutes or so and spectators are streaming in, the sun has burst through the clouds and this feels like the place to be.

There are increasingly desperate cries of "Come on Jo!" from the British contingent, between gasps as Krunic crunches her groundstrokes, and this is a big test for Konta now if she is to survive day one.

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

Krunic needs to be able to back these sort of matches up, if she does beat Jo will she be able to perform like this again on Wednesday? She is playing higher than her ranking today but she is ranked where she is because she does not play like this week in, week out.

Jo Konta needs some inspiration from somewhere. And she needs it quick.

Has she found it? A backhand winner gives the Briton two break points, but Krunic sees them off - and then wipes out a third in deuce. Konta responds on a game point for the Serb with a stunning backhand down the line.

Krunic's head is in a spin, perhaps sensing that a huge victory is on the horizon, deciding to go with an ambitious drop-shot which Konta whacks away to peg the Serb back.

Joel Newnham‏: I love an upset but not when it's a British player. #comeonjo, external there's still time to turn this around.

Image source, Getty Images