An excellent rally ends with Osaka smashing her forehand wide to give Keys a fourth break point.

But the Japanese constructs an excellent point and opens her shoulders to hit a winner across court to bring back deuce.

She takes the advantage and breathes a sigh of relief as Keys' backhand goes wide.

Smart serve from Osaka into the body. Second break point saved.

"Come on!" she shouts.

And she saves the third as her wide second serve is pushed into the net by Keys.

Deuce.

One saved...

Osaka looks nervous. She's jumping about like a kid needing the toilet.

Osaka hits a loose forehand into the net and she turns to her coaches box and slumps with frustration. She's down 30-0...

Her second serve is then completely swallowed by Keys and a thumping forehand brings up three break points.

Billie Jean King is in the house people!!

The crowd roar with excitement and even Madison Keys pauses to applaud during the changeover.

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

I think Madison Keys can reach a lot of semi-finals but the hurdle of getting to finals and winning is the thing she has to overcome. She has done well not to get injured.

A terrific backhand from Osaka as she moves around the court before whipping one into the corner.

Then a sliced backhand from Keys drops into the net. 15-30 to Osaka.

Keys draws level with a nice forehand onto the line and moves ahead with an approaching smash.

And another thumping forehand winner from Keys completes the hold.

A few loose forehands from Keys drop into the net and the 20-year-old holds serve with a 103 mph first serve out wide.

Confident stuff.

Keys' quarter-final was hugely interrupted by noise in the stadium and the closed roof is already keeping the volume in.

You can hear the rustle and chatter of the fans. But it doesn't seem to be fazing the players just yet.

Keys' second serve out wide is played across court by Osaka but the American whips a forehand around the net.

She then miscues a volley into the net before smashing a forehand winner down the line.

Osaka's next return goes wide of the tramlines but Keys' backhand does the same.

The 14th seed wraps it up with a smart serve down the middle.

A good start.

The second semi-final of the US Open is underway. Keys to serve first.

It's only Serena Williams waiting in the final...

*Denotes the server*

Despite injuries holding her back on the WTA Tour this season, Keys has stepped up in the major tournaments by reaching at least the quarter-finals in three of the four Slams - Angelique Kerber is the only other woman to achieve the feat this year.

"I feel like last year a lot of my matches were kind of emotional roller coasters, and this year I feel like I have been much more solid mentally," said Keys.

"In tougher times I'm keeping my cool a little bit better and not having to have the big emotional swing to kind of try to get back into matches.

"I'm a lot less emotionally drained this year."

Image source, Getty Images

Naomi Osaka: "I feel like I am little bit better [than the previous round], maybe there is something internally but I am feeling better.

"I think I need to have fun. She is a really good player and I know we are going to have a good match."

Madison Keys: "There will be a lot of emotions, nerves and I have to embrace it and have fun.

"We play similar, are big hitters and all that. I know any ball I hit will come back just as fast."

The two players have had their pre-match chat with ESPN too.

Keys even admitted she was a little nervous...

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

It seems to me Madison Keys is a little bit more nervous than Osaka.

The players are coming out onto court now. Madison Keys has a real 'game face' look going on.

Naomi Osaka has her headphones in. She's in the zone.

Keys is the home favourite but the New York crowds have fallen in love with Osaka this week.

And who can blame them? She's quirky, excitable and innocent.

It will be interesting to see how the fans respond during the game...

Monty Moncreiff: Wow! Serena Williams back in the US Open final for the 9th time. I don't think I've ever seen her play so well, and that's saying something! Now, can I stay awake for the second semi-final on UK time?

Keys has experience but on the other side of the net is one of tennis' brightest young stars - 20-year-old Naomi Osaka.

She is making her first appearance in a Grand Slam semi-final and she is the first Japanese woman to reach the last four for 22 years.

"It means a lot but I was much more emotional for the quarters and now I feel like it's sort of business again," Osaka said.

"Quarters was my mental goal, every time I played a Grand Slam.

"And then after I went into the quarters, I want to keep going, so I feel like I have to be focused again and keep trying really hard."

Image source, Getty Images

This time last year, Madison Keys reached her first ever Grand Slam final with victory over fellow American Coco Vandeweghe.

She would go on to lose the final to another American - Sloane Stephens - but it propelled Keys' career.

The 14th seed has since made the quarter-finals of the Australian Open and the semi-finals of the French this year, before falling short in the third round of Wimbledon.

And now she hopes to go one better by reaching another US Open final and winning her first Grand Slam title.

Image source, Getty Images