The break points keep on coming. Bacsinszky earns two more when Ostapenko can't pick up a drop-shot and sticks a backhand into the net.

A backhand winner draws the Latvian level and she beats Bacsinszky all ends up again - this time with a forehand - to nudge 40-30 ahead.

The Swiss wins the next point to take us into deuce, but Ostapenko hangs on again.

Some gutsy holds in the opening half an hour, but the bottom line is this: we're all square and still on serve.

Image source, Getty Images

Timea Bacsinskzy's relationship with tennis has been rocky over the years but, after getting her career on track in recent years, the Swiss has the edge over her young opponent in terms of experience and pedigree.

But will that be enough?

The 30th seed reached the French Open semi-finals for the second time, having been beaten in the last four by Serena Williams in 2015, with a 6-4 6-4 win against France's Kristina Mladenovic.

Bacsinszky almost quit tennis under the pressure of what she called a "control freak" father in 2013 but is now a win away from her first Grand Slam final.

"I think the magic has been happening here for me here at Roland Garros always, like, even the first time I came here as a little kid," she says.

Neither player is waltzing along yet. Ahem.

Bacsinszky tries to Quickstep to an easy hold, but it takes two to Tango. That's it now, I promise.

What I'm trying to say - if you can see through the ballroom dancing puns - is that Ostapenko makes Bacsinszky dig deep for another hold.

The Latvian takes her to deuce, but Bacsinszky hangs on.

Piers Newbery
BBC Sport at Roland Garros

"I'm playing quite aggressive. If I have a chance to go out to the net, I'm going to the net and I just finish the point, because I think it's easier."

Ostapenko revealed her simple game plan after her quarter-final win, as well as her favourite pastime - ballroom dancing.

"I was just doing some, like, Latin championships in ballroom dancing. I'm still doing it now just for myself. My favourite one is the samba."

Image source, Getty Images

A drop-shot from Bacsinszky from the baseline has Ostapenko scrambling towards the net at 30-30, she makes it...but the Swiss beats her with a perfectly-weighted lob.

Another break point...

A backhand winner from the Latvian leaves Bacsinszky wafting at fresh Parisian air, and the Swiss misses the target with two aggressive forehands on Ostapenko's second serve.

It was tough, but a morale-boosting hold is what Ostapenko has got.

Image source, EPA

Zoe Sumra: Bacsinszky and Wawrinka shared a coach (not at same time) - that might be why you see a similarity in their backhands.

Image source, Getty Images

Jelena Ostapenko became the first teenager to reach the French Open women's semi-finals since 2007 with a shock win against former world number one Caroline Wozniacki.

The unseeded Latvian - who is not a teenager any more after turning 20 today - lost the first five games of her first major quarter-final before winning 4-6 6-2 6-2.

Ostapenko also beat 2011 US Open winner Sam Stosur on her way to the last four, meaning she is now one more win away from becoming the first unseeded woman to reach a French Open final since 1983.

"When I came here I didn't expect I'm going to be in the semis, but I was playing better and better every match. If I keep it up, anything can happen," says the world number 47.

What you get with Ostapenko is plenty of winners and plenty of unforced errors. Gung ho.

The world number 47 tears into the Bacsinszky serve as she looks to hit straight back, cracking a forehand winner to go 15-0 ahead.

An aggressive return forces Bacsinszky into a poor return...break back point.

And Ostapenko has got it! Back on serve...

Ostapenko struggled at the start of her quarter-final against Caroline Wozniacki, losing the opening five games before fighting back.

Her serve comes under pressure instantly here too, Bacsinszky racking up two break points in deuce.

The first falls when the Latvian blasts down a service winner, but she can't save the second.

Bacsinszky ups the ante and Ostapenko, screeching loudly as she thumps back every ball from the baseline, knocks a forehand into the tramlines.

Early advantage for the Swiss!

Image source, Reuters

What is it with these Swiss players and beautiful backhands? Fed, Stan, Bacsinszky... They are timed to perfection.

A couple of backhand winners put Bacsinszky 30-0 ahead on Ostapenko's serve, before the not-teenage-anymore Latvian responds with a couple of winners of her own - one off each side - to go 40-30 in front...

Image source, Reuters

Rather disappointingly, there wasn't a quick rendition of 'Happy Birthday' before we kicked off. Miseries.

Bacsinszky might be higher in the rankings, but Ostapenko goes into this match on the back of a better year to date.

The Swiss starts confidently on serve though, swishing a backhand down the line for 30-15, a couple of wayward returns from Ostapenko sealing the hold.

Image source, Reuters

Right then. Ready?

Timea Bacsinskzy has a couple of balls lobbed her way by the ball-kid. She is going to open up the semi-final...

*denotes next server

Happy birthday to you,

Happy birthday to you,

Happy birthday dear Jelena and Timea

Happy birthday to you!

Image source, Google

Let's hope all those lucky punters on Chatrier have brought their singing voices. Because, in a strange quirk of fate, both of the first semi-finalist are celebrating their birthdays today.

Ostapenko is 20 today, while Bacsinszky turns 27. One is going to be celebrating later, the other is going to have their party well and truly pooped.

This year's French Open is a "land of opportunity" with no Grand Slam champions left among the women's draw, says Lindsay Davenport.

Simona Halep and Karolina Pliskova are the only semi-finalists to have reached a Grand Slam final before.

"It is wide open," said Davenport, a three-time major winner. "From two through 18 [in the rankings] it seems like there's not that big a difference."

So here's the plan on Court Philippe Chatrier today:

Jelena Ostapenko (Lat) v [30] Timea Bacsinszky (Sui)

[3] Simona Halep (Rom) v [2] Karolina Pliskova (Cze)

Equally importantly, I hope Piers has got some factor 20 suncream for his bonce.

Piers Newbery
BBC Sport at Roland Garros

It's a belter of a day for some tennis, around 25C, a gentle breeze, deckchairs out - it's more Eastbourne than Paris. Word is we might be seeing temperatures break 30C at the weekend, so energy conservation could be key if any of the four semi-finalists can get through quickly.

Image source, BBC Sport

Jelena Ostapenko walks out to a great reception from Chatrier, looking almost shy as she timidly waves to the French crowd. Timea Bacsinszky follows a few seconds later, chunky headphones blasting out some last-minute adrenaline- pumping tunage.

Wonder what she is going for? A bit of Oasis?

Bonjour mes amis! And welcome to the business end of Roland Garros.

Of course it has all been business over the past fortnight, but this is the really serious stuff.

The sort of stuff which can lead to sleepless nights, confidence crises and unpaid bills if you don;t get it concluded.

OK, that's an exaggeration. But you get the picture.

Today is a big day for Simona Halep, Karolina Pliskova, Jelena Ostapenko and Timea Bacsinszky.

Two of these women will reach the French Open final. One will become a Grand Slam champion for the first time.