Mary Pierce
Former French Open champion on BBC Radio 5 live sports extra

"This is Kiki's chance of a lifetime to make a Grand Slam final. She really has to take advantage of conditions today and this first set is very important."

Bertens is doing her bit to stay in the set - she's staying calm and moves to 30-0. 

Make that 40-0 as Williams tries to slice over but the ball falls on the wrong side. 

The American pummels a stunning backhand winner to make it 40-15 but then hits long to take us to a tie break. 

Image source, Getty Images

Mary Pierce
Former French Open champion on BBC Radio 5 live sports extra

"Serena has an opponent she should beat. Kiki has an injury and has never been to this stage before. Serena is not playing to her usual standard but her experience can be the difference."

Williams starts her service game with a booming ace. Very nice indeed.

And she adds another to move to 40-15. 

Williams tries a drop shot, and Bertens runs to respond and pokes the ball over the net. 

But another Williams ace sees her hold and take the lead for the first time. 

Mary Pierce
Former French Open champion on BBC Radio 5 live sports extra

"Kiki wanted to chase down a drop shot but slipped and that won't help the hamstring. Serena always seems to find a way of getting herself out of trouble - that's one of her greatest qualities, that she never stops fighting."

It's tense, it's close. 15-0, 15-15, 30-15, 30-30. 

Serena Williams dinks a very delicate drop shot over to earn break point.

And she does break! The world number one levels after Bertens sprays the ball wide. 

Parity. 

Image source, AP

Mary Pierce
Former French Open champion on BBC Radio 5 live sports extra

"Gabrine Muguruza making her first Grand Slam final last year gives you good experience for the next opportunity. It will help her if she gets to the final here because she will know how to deal with the emotions."

Gabrine Muguruza is a set up and a break up in the second against Sam Stosur in the other semi-final.

Williams saves break point to go to deuce, earns the advantage, and holds out after Bertens goes long.

The unseeded 24-year-old will now serve for the set. 

Good news - Serena Williams is grunting again which is always a good sign of her fired-up mentality. 

Bad news - Williams is still limping and hobbling about between points.

And Bertens has moved to break point and set point. 

Image source, Getty Images

Kiki Bertens cruises the first two points of her serve but messes up with a double fault. 30-15.

She pulls out a wonderful double-handed back hand out of her box of tricks and goes on to wrap up the game. 

A little nod to her box shows the Dutch woman is happy with that.

Image source, AP

Spanish fourth seed Gabrine Muguruza has taken the first set against Australia's Sam Stosur in the other women's semi-final over on Suzanne-Lenglen Court.

Pretty straightforward stuff - 6-2.

Risky business from Bertens. She's at 0-15 when she decides to leave a Williams lob and it goes over her head, only to bounce well in. 

Williams makes sure she doesn't slip up again in this game. She brings it to 3-4. 

Kiki Bertens doesn't look like a debutant in a Grand Slam semi-final. She's so relaxed as she bounces about the court. 

She goes into a 40-15 lead, but is pegged to 40-30 by a backhand winner from Serena Williams, who makes a rare bolt for the net.

Bertens does wrap up her service game, though. A small fist pump for you, young lady. 

Image source, AP

Andrew Duncan: Women's semi-final played out in half empty stadium. Should move it to Spain!

There's an awful lot of flooding in Paris at the moment, which may contribute to the lack of people...

We wobble on deuce, advantage Williams, deuce, advantage Williams.

Eventually the defending champion holds her serve after Bertens hits into the net.

Williams genuinely looks like she's about to burst into tears. She's gone for a sit down. 

Bertens has two more break points after finishing off a fantastic rally with a wonderful forehand winner. 

But we go to deuce after the Dutch woman twice overcooks a return. 

With a roar of rage, Williams takes the advantage with an overhead smash, before returning to deuce again after another Bertens forehand winner.

Mary Pierce
Former French Open champion on BBC Radio 5 live sports extra

"If you have an injury you don't want to show anything to your opponent - but there are no signs from Kiki that she's struggling. Serena is not looking her usual intense, fiery self."

Image source, AP

Kiki Bertens, ranked 58 in the world, races into a 40-0 lead, with Serena Williams returning all over the shop.

And she holds to love as Williams swipes into the net.

This is a worry for Williams. She looks out of sorts, and there's been suggestions she's struggling with a thigh problem. Let's hope it's nothing too serious. 

Williams racks up her fifth unforced error on the first point of her service game. Eeek.

A powerful backhand straight down court brings her to 15-15. Looks like the American has warmed up a bit now. 

She blasts a forehand winner beyond her opponent but Bertens responds in kind to take it to deuce.

Williams eventually sees out the game after Bertens hits long past the baseline. Relief for the world number one. 

Mary Pierce
Former French Open champion on BBC Radio 5 live sports extra

"Wow. Kiki is playing really well. She looks calm and focused. The drop shot was the right shot there."

Image source, AP