As Piers says Mladenovic has now hit 22 unforced errors to Bacsinszky's 12.

There are two more to the Swiss player's total though, as she hands a couple of points to Mladenovic to take us to 30-30.

Mladenovic has a look at another break point but can't take it and some good serving sees Bacsinszky clinch the opener.

That was a total mess of a set but it is world number 31 Bacsinszky who is a set from the semi-final.

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Ah. Anything you can do Jelena...

Wozniacki instantly produces a fightback of her own from 30-0 to break back instantly.

There are more breaks than a Kit Kat multipacks in this match...

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Someone bring a hose. Jelena Ostapenko is on FIRE!

Wozniacki is cruising towards a comfortable hold when Ostapenko decides enough is enough.

She fights back from 40-0 to break the Dane's serve at the first opportunity in this second set.

Ladies and gents, we're pleased to announce that we have a game on our hands.

Image source, Getty Images

Piers Newbery
BBC Sport at Roland Garros

The stats are never likely to be kind on a day like this, it's about keeping the ball between the lines and hoping for the best, but Mladenovic has leaked half a dozen more errors and won just 36% of points on her second serve so far. That could make the difference in this first set.

Brilliant from Bacsinszky. This set has been tough on the legs and she takes another marathon point with a big backhand winner and then has another in the bag on her fourth break point of the game.

The Swiss player is ahead again. She'll serve for this first set but there are absolutely no guarantees.

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A big let-off for Mladenovic as she comes into the net behind her serve, plays a poor volley but Bacsinszky nets.

What a point! Two or three times it looked over with both players scampering front and back left and right but Mladenovic takes it. Both break points saved.

Twice the French woman is a point from taking the game but again plays drop shots which are mopped up by her opponent.

We've now played over nine minutes in this crucial game...

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Mark Fletcher: If it's as windy as the text suggests Novak won't be happy. Upset on the cards for Thiem?

Now this opening set is all over. Jelena Ostapenka showed remarkable determination to claw Caroline Wozniacki back, only to lose grip of the Dane at the vital time.

And it is probably her worst service game of the set.

Three unforced errors and a double fault help Wozniacki over the line.

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It is getting a little drop shot crazy out there, both players not willing to unleash their full ground stroke power in the wind.

Mladenovic is the guilty party this time and it sees her in trouble as Bacsinszky has another two break points...

'Kiki' Mladenovic knows how to play a home crowd. She never gives up and is showing off all of the fist pumps to get the fans on side.

The 13th seed breaks back for the second time in this set. Her support team are on their feet.

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Who'd have thought less than 20 minutes ago that this opening set would still be going? Wozniacki was 5-0 ahead remember...

Has the wind on Lenglen dropped a little? Possibly. While the flags are still flapping ten to the dozen, the players' skirts aren't swishing as much and there certainly isn't any clay flying around now.

Ostapenko has settled down too, cutting down on the unforced errors and landing more winners.

That helps her clock up two break points, which she blows, but she is playing with renewed confidence...

Another chance comes and goes in deuce and then, finally she gets over the line when Wozniacki slaps a backhand into the net from the baseline.

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Ooooooh. There's the first time violation warning from Carlos Ramos in the umpire's chair as Bacsinszky tries to buy time for the wind to drop before serving.

What a shot! Finally the tennis in this match is starting to match the occasion as Mladenovic beats Bacsinszky with a passing shot.

She wins the next point with a nervy looking volley at the net.

Deuce. "Kiki, Kiki," shout the crowd...

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The home crowd now sense their player needs a helping hand.

At 30-15 a round of applause rings around Chatrier to encourage the 13th seed Mladenovic.

Error number 11 from Bacsinszky is followed by some good hitting from Mladenovic and the crowd have what they were after, another game for the French number one.

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Bacsinszky is hitting with plenty of spin and slice and showing all of the skill and match-play experience that took her to number nine in the world this time last year.

She hits a great backhand drop shot that gets a hum of appreciation from the crowd en route to taking the most comfortable game of the match.

Image source, EPA

One break down, one to go...

Ostapenko shrieks an ear-splitting "Come onnnn!" as she nabs a Wozniacki service game back with a thumping backhand winner in deuce.

No wonder the teenager is relieved: she blew two break points in that game before finally, finally taking her eighth chance of the match.

Apparently the Latvian used to be a bit of a ballroom dancer. She's certainly found her rhythm and fancy footwork to stop Wozzy waltzing to the opening set...

Image source, AFP

Piers Newbery
BBC Sport at Roland Garros

The Mladenovic box is by far the fullest I've seen all tournament, six rows of Team Kiki packed in there, and we've had an early chant of "Kiki! Kiki!" from the shivering crowd. This is going to be a tough day for players and spectators alike (the forecast for the rest of the week is good, thankfully). I did not expect to see any Parisians in a puffer, hoodie and woolly hat combo this week.

Five games, three breaks of serve.

Mladenovic, hitting into the gale, tries another drop shot but this one isn't hard enough.

Bacsinszky restores her break advantage but I don't think that means too much in these conditions.

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Just like the players, the crowd have struggled to get into this match so far.

A raucous atmosphere in Mladenovic's last match saw defending champion Garbine Muguruza leave court wagging her finger at the locals as a result of the partisan support of their player.

The biggest cheer so far comes as a Mladenovic drop shot dies in the wind but makes it over the net, giving her a winner. Bacsinszky is battling though, showing her three years' greater experience and brings up another break point...

Image source, EPA

Latvian teenager Jelena Ostapenko is on the board. A morale-boosting hold for the Grand Slam quarter-final debutant is greeted by a warm round of applause from the Lenglen crowd.

Can that spur her on? She needs to break Wozniacki's serve in the next game to avoid losing the set.

Image source, Reuters