Victoria Azarenka tightened up horribly on the brink of victory over Williams in Madrid earlier this year - chucking in three double faults in the final game to allow Williams to take her into a tie-break and then curtains.

No signs of the gremlins getting out of their cage just yet. Another solid service game that Williams cannot put any big dents into.

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Serena Williams as vulnerable as a peach in a blender at the net as Victoria Azarenka has time and an inviting mid-court bounce to look at at 15-40.

Williams makes the volley, with a helpful bit of graphite in the shot shooting the ball off at an awkward angle and killing it dead.

A forehand thump saves the second. Is that the old Williams stirring? Usual service resuming?

Afraid not. Azarenka is gobbling up these serves, reading them like a summer beach novel and a walloped return winner moves her a set and a break up.

Williams needs to pull the ripcord as soon as possible. She is in freefall now.

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Serena Williams trying to force it, wrenching at some groundstrokes and over-cooking beyond the baseline.

There is a bit of tension in those arms. She knows what she wants to do, but is struggling to fine the rhythm she requires.

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More like it from Serena Williams.

An ace kicks her onto 40-0, before she hammers away a smash to secure the game.

There looks like there is more fire in her belly now that the scoreboard slate has been wiped clean as well.

She bellows in delight after putting away the final shot.

Piers Newbery
BBC Sport at Roland Garros

"A pretty impressive opening 40 minutes from Azarenka but Williams is struggling to find her game, just as she did in the previous round. It seems she might be suffering with a cold like Sharapova, apparently asking for tissues at one changeover.

"Something would certainly seem to be up when the best server in the world is winning just 45% of points behind her serve."

Serena Williams saw that serve like a beach ball. A whip of the torso and the ball zooms away to the sideboards, kissing the whitewash en route, for 15-30.

But Victoria Azarenka is just not missing. Constant pressure on Williams, shifting her left, right, up and down the court and it crumbles her defence.

First set bagged.

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Serena Williams wraps up her own serve tight.

Now she is going to have to come out roaring against an uncowed Victoria Azarenka. The Belarusian serves for the set next.

Victoria Azarenka is making a mockery of her 27-in-the-world ranking.

She races into a 40-0 lead with the third point secured with a superb instinctive backhand jab winner that was all touch and feel. Pure hands.

Williams nibbles back a couple of points but is wiped out of the game by a monster serve-crashing volley combo.

Williams had problems against unheralded German Anna-Lena Friedsam, before winning in three sets in the last round. This is a test of a whole higher order.

Victoria Azarenka takes a leaping forehand right out of the Nick Kyrgios playbook to club a return winner for 15-40.

Oh, and that is a lovely little tickle.

A back-spinning backhand drop-shot makes it over the net, with the aid of a small hop from Azarenka, and dies door nail dead before Williams can get close to running it down.

Delightful and fully deserving of the break for that shot alone.

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Victoria Azarenza is as tenacious as a terrier and refuses to be shaken off by Serena's power.

The Belarusian shows that she can match the pace coming back from the opposite end and find some awkward angles into the bargain.

She holds with a bit of a struggle. By the way Sloane Stephens has beaten Tsvetlana Pironkova and will play the winner of this one.

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Piers Newbery
BBC Sport at Roland Garros

"The shadow has already crept from the tram line to the 'T' since they players arrived on court and Azarenka might just want those trousers back in half an hour, it could get chilly.

"The points are brief, with both women going for it at the first sign of a short ball, and the whole contest has a very serious feeling about it already. No messing out there, two title contenders with their hopes on the line."

Serena Williams's first double fault is followed by a canny dart into the net from Victoria Azarenka - patting away her volley for 30-30.

Oh, that is lovely. Victoria Azarenka rolls her wrist over a forehand to lift the ball, up, over and in to set up another break point at 30-40.

But this is Serena is cartoon superhero mode. Blammo! Biff! Bosh! String-stretching service power shuts the door firmly in Azarenka's face.

Serena Williams is strolling around the court, belting the ball and producing a fairly even mix of winners and dross.

She squanders the first break point from 15-40 with an unforced error, but a firehose barrage of pace and power forces Azarenka long to tie down the second.

Back on serve.

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Now then, Victoria Azarenka with a look at 0-30. And now a devilishly deep return helps her to 15-40 and two break points.

Serena Williams twangs the wire in the next rally. It loops up and lands in, back and forth they still trade. That one is staying south though!

Williams dumps into the middle of the net and Azarenka has a break at the first opportunity.

Serena Williams welcomes Victoria Azarenka to the match by clattering a huge return winner off the Belarusian's second serve on the very first point.

Not a flicker of reaction from Williams. She strolls across to the other side of the court with a poker face, having firmly slapped down the gauntlet.

Azarenka moves on up to 40-15, pegged back to 40-30, and then a delicious angle off a backhand punch from Williams takes us into the deucer.

Williams pulls a big theatrical shrug - all upturned palms and jutting hips - after an Azarenka serve spits off the dirt.

Azarenka's serve is being hunted down by Williams, but after several attempts she finally wraps up a game point. That was a struggle against a pumped-up Serena though.

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The players are out on court.

And in a bold sartorial decision/canny tactical move, Victoria Azarenka is warming up in a pair of grey trousers.

I guess in most other sports the players warm up in several layers. Why shouldn't tennis players try and get a flying start with warm hammies?

Azarenka to serve...

It's our first taste of two of the top guns going at each other in Paris this year, and it should be a tasty affair.

But can Victoria Azarenka topple Serena Williams at Roland Garros? Or will the American charge all the way to the title?

Give us your thoughts and get involved using #bbctennis on Twitter

Piers Newbery
BBC Sport at Roland Garros

"On a day of big names, this always had the potential to be the biggest match. The world number one against a former number one, making her way back from injury and at a very misleading 27 in the rankings.

"Victoria Azarenka had been the woman most likely to challenge Williams until a foot injury wrecked her 2014, but the Belarusian showed she was back in Madrid earlier this month. At 6-5, 40-0, in the final set, she had Williams at her mercy. Then she blew it. Paris would be the perfect place to go one point further."

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Nineteen Grand Slam singles titles, current world number one, two-time Roland Garros champion. There is no doubting Serena Williams' credentials.

Victoria Azarenka though has an impressive resume of her own for the on-court announcer to build the hyperbole with.

A two-time Grand Slam champion, 51 weeks at world number one and a former semi-finalist in Paris. She has made at least the last four of all the big four events in the tennis calendar.

Injury is the only reason for her ranking being down at its current level of 27 in the world.

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Sometimes, with the vagaries of ranking points, you get a Grand Slam match in the first week that belongs in the back end of the fortnight.

This is one of those matches.