Osaka's serve is out of reach of Keys but the tip of the American's racquet connects with the ball. It flies into the crowd and Keys apologises for hitting someone on the front row.

It's relentless from Osaka though, as she continues to push Keys deeper and deeper.

Keys responds with a rippling forehand winner before Osaka serves a 116 mph ace down the middle.

30-40 to Osaka.

Slick.

Osaka reads the serve, slams it back in play and Keys hits it long. A challenge is needed to confirm it's out and when it is, Osaka turns on her heels with a bounce and bounds with confidence to the back of the court.

Keys looks uncomfortable. She starts the second set by blasting a forehand over the baseline.

And Osaka gets the break point as Keys leans back and sends her forehand long.

Jeff Tarango
Former professional tennis player on BBC Radio 5 live sports extra

Keys needs to stop being so negative. She needs to move her feet better and cut off the angles. She has to think about her depth because when she keeps trying to add interest to every ball you can get behind in the rally.

A very good opening set from Osaka. Her statistics outshine Keys in all the right departments.

70% of her first serves were in play, compared to 59% of Keys. And her second serve success rate is 11% higher than Keys.

Osaka also won 2/3 of her break points, while Keys squandered all 6 of hers.

Keys' return is straight into the stomach of Osaka and she can't get her feet around the ball. Deuce.

A sublime rally breaks out - Keys running from side-to-side, returning forehands at acute angles and pushing Osaka to the limit before the American eventually swats it into the net.

Osaka then smashes a forehand and takes the first set.

The Japanese star looks nervous. And it shows as she slots a forehand into the net to go 30-0 down.

A nice change of direction and a smart cross-court forehand gets her on the board and she responds with a fist pump and another "COME ON!"

Keys whips a forehand into play but Osaka matches it with another deep ground stroke before pushing Keys wide with her forehand.

Set point.

Osaka to serve for the first set.

Russell Fuller
BBC tennis correspondent on 5 live sports extra at Flushing Meadows

That sort of game really worries you if you are Madison Keys because we have seen it before. Her game can unravel.

Gigi Salmon
Radio 5 live sports extra commentator at Flushing Meadows

All Osaka had to do was get the ball in play!

A soft forehand from Keys, again, and it's game and double break for Osaka.

0-30 already. Keys doesn't look positive.

Another slice hits the net. Coach Lindsay Davenport looks worried.

And Keys goes long with her forehand. Three break points for Osaka.

And breathe...

Keys goes ahead with a sliced return. A big serve from Osaka saves break point.

Then Keys hits into the net and drags her forehand long.

Osaka holds and the break is consolidated.

Double-fault from Osaka... Break point to Keys.

Then an ace down the middle from Osaka. Back to deuce.

She takes the advantage as Keys returns her wide serve into the net but Osaka is pushed back on the next point and her forehand falls short.

Deuce again.

But it's in! 40-30 to Osaka.

Keys is still battling and she engages in a heavy-weight rally before coming out on top with another thumping winner.

Deuce.

Osaka is creating some nice angles. It's forcing Keys to slice and another one hits the net.

But the American uses her powerful ground strokes to attack Osaka's second serve and hits another forehand winner.

The next one is too deep though and Osaka draws level. 30-30.

Osaka tries to push Keys wide again with a cross-court forehand and Keys has challenged it. She thinks it's out...

Gigi Salmon
Radio 5 live sports extra commentator at Flushing Meadows

You can play fantastic tennis. But if you're out there and there's 22,000 people and you're nervous. What on earth would you do?

It feels the weakness is Keys' nerves in the big moments.

Osaka's return hits the net but she gets the break at the second time of asking.

She absorbs the pace of Keys' forehand and hits it deep towards the baseline before the American smashes it into the net.

Two break points for Osaka...

A couple of loose shots from Keys. She's just missed five break points of her own. How costly will it be?