Madison Keys to tee off...

And she starts with a service winner.

Dennis Ethan: I want a three-set match with Vandeweghe winning the final set

In for the long haul then, Dennis...

Jill Craybas
Former world number 39 on BBC Radio 5 live sports extra

I always thought American tennis is very cyclical, at one point all the top 10 players were Russian and then they fell off. It takes some time, it doesn't happen straight away.

There has been a lot of attention on Madison, Coco and Sloane. For me it was a matter of time for these three youngsters coming through.

This group of girls, they really do support each other. The joy of Madison and Coco being in the Stanford final together. That is one of the biggest differences in why they've been successful.

Image source, Getty Images

There is some formidable sporting heritage floating around in the Vandeweghe gene pool.

Coco's mother Tauna (pictured above) represented the USA at the Olympics for swimming in 1976 and volleyball in 1984.

Her grandfather Ernie played for New York Knicks in 1950s while her uncle Kiki was general manager of the Denver Nuggets and a former UCLA basketball player.

Added to which her grandmother was Miss America 1952, but I don't think the swimwear round involved a lot of actual swimming.

Jeff Tarango
Former professional tennis player on 5 live sports extra at Flushing Meadows

I'm cuckoo for Coco. I think Pat Cash has made a huge influence not only in her game, but in her life. She is walking out there so cocky, so brash, I don't think she even thinks there is a loss in her future.

Image source, Getty Images

This is not Madison Keys' first Grand Slam semi-final rodeo.

The 22-year-old reached the last four of the 2015 Australian Open where she was beaten by eventual champion Serena Williams.

And Coco Vandeweghe got her first taste of a big four semi-final earlier this year in Melbourne.

She ran into a different Williams sister, but with the same outcome.

Image source, Getty Images

Lindsay Davenport must be comfortable in her seat in Madison Key's camp as she served up this little bit of social media salt in the run-up to the US Open...

App users may need to click through to view Davenport's tweet.

Image source, Getty Images

As is often the case nowadays, the coaches in this match are perhaps better known than their charges.

In Madison Keys' corner is three-time Grand Slam champion Lindsay Davenport. The pair have been together for nearly three years which is pretty steady going in the chop-change world of lockerroom relationships.

Backing Coco Vandeweghe is Pat Cash, of checkerboard-headband and Centre Court-scaling fame.

"I think the biggest thing is channelling my intensity and tenacity out onto the court and putting it into a singular focus," Vandeweghe said of their partnership.

"You will have to ask him how he's been able to do that. I don't really know. Maybe it's like some Jedi mind trick."

Image source, getty

Image source, Getty

Both of this pair's previous meetings have been within the past few weeks with Keys winning the third title of her career with a 7-6 (7-4) 6-4 victory over Vandeweghe in the Stanford final last month.

It doesn't look like there were any hard feelings as the pair shared a seat ahead of the trophy presentation.

A fortnight later Keys won again when they met in the first round in Cincinnati, this time over three sets, 6-4 3-6 6-3.

Image source, Getty Images

Cheers Alex.

How to follow that?

Well, how about more of the same?

Two well-matched American players and intriguing characters going toe-to-toe as Madison Keys and Coco Vandeweghe prepare to walk onto Arthur Ashe.

Right then, after that epic, we've still got another all-American semi-final to come. And for that, I'll hand the baton over to Mike Henson...

Sloane Stephens gets congratulations from Oracene - Venus and Serena Williams' mother.

Jill Craybas
Former world number 39 on BBC Radio 5 live sports extra

I think Venus played an unbelievable match, but Sloane just lifted her level in the third set, with superb defence and great shots from difficult positions. I think that frazzled Venus because she is used to winning those rallies.

Image source, Getty Images

Sloane Stephens: "I have no words to describe my feelings and what it took to get here. I have no words.

"I have no idea how I have done it. Your guess is as good as mine. If someone had told me when I started my comeback that I would make two semi-finals and a Grand Slam final I would have passed out. I don't know how I did it. Just hard work.

"I hung tough, gritted it out. I worked my tail off. We played some incredible points in the third set.

"I am honoured to play at the same time as one of the greats of the game. She is the most graceful, the most elegant."

Jeff Tarango
Former professional tennis player on 5 live sports extra at Flushing Meadows

Three of those points will go down in the record books as greatest points in a US Open semi-final. I've never seen her play such good defence, and just when it looked like her forehand had gone passive she stepped it up.

Venus had no answers in that last game.

This match has had American Tour player Irina Falconi gripped...

App users may have to click through to view this tweet.

Venus Williams leaves the court to a standing ovation from the Arthur Ashe Stadium, before disappearing down towards the locker room.

As for Sloane Stephens, it's her moment in the limelight...

Image source, Getty Images

Russell Fuller
BBC tennis correspondent at Flushing Meadows

That is a famous famous victory beating a two-time champion in a third set that will live long in the memory.