Andrea Hlavackova
Grand Slam doubles champion in Paris on BBC Radio

When I arrived here the girls in the locker room were saying Naomi Osaka will win a Grand Slam. Maybe, I don't know, there are so many good girls in the next generation coming up. But she is definitely one of the good ones. If she stays healthy and performs then she has got some potential.

Image source, Getty Images

As Piers alluded to Rafael Nadal is next on the Arthur Ashe ramp.

The Spaniard, who returned to world number one last week, is up against Serbian world number 85 Dusan Lajovic.

The last time they met - back in 2014 at Roland Garros - Nadal gave up just four games over a three-set victory.

Andrea Hlavackova
Former Grand Slam doubles champion on BBC Radio 5 live sports extra

I think the world number one ranking put a blanket on Angelique Kerber. She didn't look like she was enjoying it.

When she lost her number one ranking I could see a bit of relief from her. She is an introvert and doesn't like the spotlight of the world.

I have known her since we were 12 years old, she is a very nice girl. But you have to get to know her and break the shell a little bit. She is a fun, nice girl but not the spotlight girl.

Piers Newbery
BBC Sport at Flushing Meadows

Osaka celebrates by smacking a few balls into the crowd and then narrowly fails to hit the roof after launching one straight up in the air - this is a big, big tennis stadium. The 19-year-old looked well at home, and having moved to the USA with her family at the age of three, has a familiar accent for the home crowd.

Not so Rafael Nadal, who is revered like a local here anyway. The Spaniard will get a raucous New York welcome, and once everyone else returns from grabbing refreshments this might well have the feel of a night match at 2.30 in the afternoon.

David Law
Radio 5 live sports extra commentator at Flushing Meadows

Image source, Reuters

It has been a horror year for Angelique Kerber. We all wondered how she would deal with being the targeted one, the world number one. It wasn't a flash in the pan last year, she won two Grand Slams finals and reached another - you don't do that by accident.

But right from the start of year she looked like somebody uncomfortable in a position of world number one. I got the sense she would prefer not to be world number one in a way - it is not her comfort zone.

She likes to be part of the pack, play really well and everyone want to be surprised how good she is. I feel a bit sorry for her because she doesn't look like she has enjoyed this year.

Image source, Getty Images

Japanese teenager Naomi Osaka on beating defending champion Angelique Kerber: "It means a lot, especially since last time I was here (blowing a 5-1 final set lead against Madison Keys). This court has not given me fond memories but hopefully this overtakes that.

"When it was 4-1 today I was thinking 'I really hope I don't do what I did last year and that let me focus a lot.

"I tend to scream a lot but I was really calm today and I am happy about that. I just really want to play good tennis and I played good tennis today. I hope I can carry that on in the next round."

This US Open is shaping up for someone to make a name for themselves....

Or for one of the old guard of Venus Williams, Rafael Nadal and Roger Federer to keep the kids in their place.

David Law
Radio 5 live sports extra commentator at Flushing Meadows

What a performance. And Osaka has done it in the most matter-of-fact way. The look on her face is like 'well, what do you expect? Of course I was going to do that'.

Angelique Kerber marches back to the dressing room, head and visor down as a cameraman tries to poke his lens into her private thoughts.

Naomi Osaka meanwhile is preparing for her on-court interview after the biggest win of her career.

David Law
Radio 5 live sports extra commentator at Flushing Meadows

Angelique Kerber looks to her team and nods after slapping that backhand into the net. There's a resigned look about Kerber. She is trying to inject pace, but that isn't her game. It is usually all about consistency.

Click the 'live coverage' tab to listen to BBC Radio 5 live sports extra commentary.

Never mind the power, feel the depth.

Naomi Osaka shows another string to her bow as she carefully guides the ball up to the baseline, right under Angelique Kerber's nostrils, to force a mistake at 30-30.

Kerber can't summon the sap for this fight either.

She folds out of the game meekly and the titanium will that marked her game 12 months ago seems to have been subbed for a weaker alloy.

Piers Newbery
BBC Sport at Flushing Meadows

In terms of rankings and experience this could be a major shock, but an Osaka win would not be entirely unexpected. Chris Evert said this was the match she was looking forward to most after Sharapova v Halep, and the Japanese player's fearless hitting is well matched to overcome a player whose brilliant defensive skills have faded this year. A healthy contingent of Japanese journalists were certainly optimistic judging by their numbers on court, while the sprinkling of German press are unsurprised. It feels a long time since Kerber was winning Grand Slam titles.

Having said that, Osaka led Madison Keys 5-1 in the final set here 12 months ago, and lost, so a few demons may still be lurking...

Anglique Kerber is left ruefully shaking her head at the back of the court, unable to compute what is coming back from over the other side.

Industrial-grade ball striking from Naomi Osaka, who is more panel-beater than portrait-painter out there.

It is working just perfectly though.

Kerber is comprehensively outgunned and is now a set and a double break adrift.

It is going to take a combination of an extraordinary comeback and a Heimlich-level choke to keep the defending champion in the draw.

Oh my.

Naomi Osaka is pummeling the life out the ball and closing the door on Angelique Kerber.

A hold to love and she whips away a delicious winning forehand on a ridiculous angle with eye-watering pace.

The Arthur Ashe crowd is patchy, but those who have made it in early are locked in.

Image source, AFP

From Ben Rothenberg of the New York Times...

Better from Angelique Kerber who holds to love.

But, you see that horse? That stable door?

It might all be too late.

Four more service holds from Naomi Osaka and the German's US Open campaign is toast.

Piers Newbery
BBC Sport at Flushing Meadows

The afternoon sessions on Ashe can be a little low-key when the sun is shining and there are generally more competitive contests elsewhere, but the giant arena is filling up and buzzing with excitement as the only match on offer right now could produce a major upset. The roof only serves to increase the volume of the communal conversation going on in the stands, meanwhile a phone goes off behind me. They answer, obviously. Dinner plans in town, apparently.

Naomi Osaka made the third round on her main-draw debut last year, but the manner of her eventual defeat, fluffing a 5-1 third-set lead against Madison Keys - raised doubts that she was hard-nosed enough.

Perhaps she still isn't.

15-30 turns to 30-40 and break point Kerber.

Or maybe this is Osaka 2.0.

Big, brassy baseline power drags Kerber out of shape and steals back the break pont.

And Osaka lassoos in a whopper of an inside-out forehand to seal the game and her comeback.

Upset very much on.

Image source, AFP

The famous Japanese 'bullet train' travels from Tokyo to Osaka and right now Naomi seems to have that sort of momentum.

The Japanese is chowing down on Angelique Kerber's second serve, that has descended from powderpuff to pattercake.

Three break points at 0-40.

And, surprise, surprise, facing a second serve at 15-40, Osaka does the bizzo.

A set and a break. Are nerves the main threat to the underdog now?