Good stuff from Caroline Wozniacki, winning a rally with a backhand that flies into empty space!

Four match points for Wozniacki to play with. She can wrap it up this time, right?

Too loose from Elise Mertens, and she flicks her backhand into the net.

A change of ends, and Mertens slashes a loose serve out of play.

Glorious from Mertens! A thudding forehand, hitting hard and straight, and Wozniacki can't get near it.

Up to the net the Belgian comes, and she works her winner with Wozniacki still sprawling from the serve.

An ugly looking rally, but Wozniacki comes through it with an overhead that Mertens spears wide.

A baby crying disturbs Wozniacki briefly, but a fine serve beats Mertens.

Deep hitting from both players, but it's Mertens who falters with a forehand into the net.

Good serve from Wozniacki, and Mertens nets the return.

Wrapped up with a cross court forehand! Hello, tie-break...

A missed opportunity from Mertens, as she steps in and launches her forehand long.

Not again! Out goes Wozniacki's backhand, and we're going back to deuce. This is uncomfortable to watch.

Wozniacki volleys, and volleys nicely to get the advantage back.

And then Elise Mertens throws the kitchen sink at a glorious forehand winner. Ooft!

On we go.

A moon ball from Caroline Wozniacki, and Elise Mertens thrashes her forehand into the net!

Would you believe it?! Wozniacki double faults again! Back to 40-40.

Second set point save! Wozniacki rediscovers her forehand and crashes into the corner of the court.

Deuce.

Wozniacki's first serve is called out, she challenges it - and it is indeed out! Ooft. Out by a hair's breadth.

Second serve, pushed out wide, and Mertens only just manages to get it over for Wozniacki to thunder the winner. First set point saved.

A strong Hawk-Eye challenge from Elise Mertens sees a call of out on her backhand overturned, before she lets out a yell as she thunders a forehand winner cross court.

A long rally eventually goes Wozniacki's way, Mertens just drifting long with a forehand, but now there's two set points to Elise Mertens!

Caroline Wozniacki's dad, Piotr, is chuntering furiously to someone in her box.

Wozniacki, wisely, pays him no attention. She focuses straight ahead, looking into empty space, before wandering back out to serve.

I actually feel a bit awkward watching Caroline Wozniacki between games. There's tension written on her face.

It's a nervy situation, not helped by a excellent love hold by Elise Mertens.

Image source, Reuters

Double fault! Oh, that is so, so unfortunate for Caroline Wozniacki.

Mertens breaks back.

An ace out wide gets things going. As easy as you like from Wozniacki.

She gets to 30-0 with ease before a crunching forehand drifts just over the line. Wozniacki's ball toss goes all wrong, though, and her nerves show as she double faults.

Oh, and that's a winner from Mertens! Wozniacki goes forward and Mertens passes her with a forehand. Break point!

Caroline Wozniacki, serving for a place in her first Australian Open final and her first Slam final since 2014, takes a deep breath as she walks to the baseline.

Here we go!

Image source, AFP

Forward comes Mertens and away goes a furious forehand volley, the ball landing right in the corner of the court.

Ooft. Wozniacki leans into a poor second serve but blazes her backhand into the net, and Mertens keeps her cool to see out a love hold.

It's a case of close but not close enough for Elise Mertens, as she takes control of a mini-rally before launching her forehand over the baseline.

Wozniacki races to 40-0 with some superb first serves, and she holds to love. She's a game away from the final.

Elise Mertens makes a horrible mess of an overhead with Wozniacki out of the picture, and she holds as an unusually nervy-looking Woznaicki cracks a forehand long.

Woz stomps back to her chair and tells herself to "calm down". The last thing she wants is a repeat of the 2011 semi-final...