Mertens just can't get into these rallies against Wozniacki.

She gets moved around a little by the Dane, before Mertens slices her backhand into the net. That opens the floodgates, and Wozniacki steamrollers through a love hold.

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Into the net goes Mertens with the backhand and, as the clock ticks over to the hour mark, Wozniacki gets the break.

Elise Mertens lets a 30-0 lead slip and, with another shanked forehand, hands Caroline Wozniacki a break point.

A scrappy game from Elise Mertens, lashing a lazy forehand long, and Caroline Wozniacki takes full advantage.

Down comes her fifth ace, and despite a backhand drifting long, the Dane completes a regulation hold.

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A smash at the net gets Mertens going, before the rarest of rare incidents occurs as Wozniacki misses a forehand.

Mertens gets drawn into a rally and again, it's Wozniacki that comes out on top, but a big serve helps her hold to 30.

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A Wozniacki backhand winner leaves Mertens utterly rooted to the spot. She didn't see that coming. An ace and a couple of netted backhands from the Belgian hurries along a love hold for the Dane.

Piers Newbery
BBC Sport at Melbourne Park

Wozniacki has been chewing up the regular pace coming at her from Mertens and it might be time to try and mix things up from the Belgian. She also needs to somehow get a hold of the Wozniacki first serve, with the Dane winning 76% of those points so far.

Father and coach Piotr Wozniacki looks on from the player box, delighted no doubt. They have tried numerous different coaches over the years but Caroline has always been clear that her father will be part of any coaching team, and in the end it always comes back to him.

Good serving from Elise Mertens, forcing the errors out of Wozniacki.

Mertens holds, but it ain't easy out there for her.

* denotes next server

A good second serve, right into the corner of the court, saves th first break point as Mertens wallops a winner behind Wozniacki.

And again! The first serve isn't clicking but the second, straight down the T, wrongfoots Woz. Deuce.

Just like that, two break points come Caroline Wozniacki's way.

A loose backhand costs Mertens first, before she coughs up another double fault.

Here we go then. Elise Mertens will serve to start the second set.

14 unforced errors in that set from Elise Mertens, compared to six from Caroline Wozniacki.

There were 13 winners from the Dane, too. Mertens has just popped off court while Wozniacki stares ahead with a look of steel.

A fine rally from Caroline Wozniacki, pinning the ball right onto the baseline, and Elise Mertens can't help but whip a forehand out as she slides to meet the ball.

Wozniacki is a set away from an appearance in her first Australian Open final.

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An overrule from the umpire means we're going to have to replay the point, which prompts Wozniacki to come in and have a grumble at the umpire.

The umpire is having none of it, though, and Wozniacki clonks down a big serve to take her to 30-0.

Good, strong serving from the Dane, and she's got three set points to play with.

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Thwack goes Mertens with the serve, and it beats a sprawling Wozniacki.

A loose forehand from Wozniacki finds teh net, and Mertens holds to force her to serve for the set.

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Oh! Elise Mertens gets Wozniacki in the perfect position, comes forward for the volley - and drops it into the net.

Set point, Wozniacki - and it's saved as the Dane's forehand drops long! Wozniacki challenges - you would, wouldn't you? - but it is indeed out.

Caroline Wozniacki comes tearing around to pick up a forehand from Mertens, deep behind the baseline, but her backhand just hits the top of the net!

Advantage to Mertens, but she can't close it out. Back to deuce we go.

Wozniacki shows that she can wallop forehand winners, as well as backhands, as the ball goes spinning beyond Mertens.

A shimmy forward from Mertens rushes her opponent, and Wozniacki loops her forehand out, but the Dane takes advantage of a poor second serve to crash her backhand over for 40-30.

Double fault from Mertens! Of all the times...

A cracking backhand down the line from Wozniacki is followed by a dart forward and a pushing forehand jab from Mertens.

A solid service game from Wozniacki, though, and she's a game away from taking the first set.

Whack! The ball makes a delightful noise off Wozniacki's racquet as she moves herself out wide again and powers a forehand in.

Mertens responds with a nice strike down the line of her own, but her forehand isn't quite clicking yet. An attempted winner hits the net, before Mertens comes to the net and volleys her way to a hold.

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