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Another break point for Aryna Sabalenka but she is denied by an ace.

Coco Gauff then has the chance to complete the hold but a double fault keeps Sabalenka in contention.

A big couple of points coming up here...

A really important first game here for Coco Gauff in a must-win second set.

But Aryna Sabalenka, full of confidence and playing more freely after taking that first set, is going to take some stopping.

The Belarusian has a break point - but boots the ball in frustration after missing a simple volley at the net.

Jonathan Jurejko
BBC Sport in Melbourne

One of the hallmarks of Coco Gauff's US Open triumph was her ability to bounce back.

Resetting after losing the first set was something she did three times - including in the final against you know who.

But this is the first time at Melbourne Park this fortnight where she has trailed.

Billie Jean King, Evonne Goolagong Cawley and Margaret Court are among those lucky to be enjoying this thrilling semi-final on Rod Laver Arena this evening.

Image source, Getty Images

Image source, Getty Images

Image source, Getty Images

Image source, Getty Images

A final big serve closes out a one-sided tie-break and Aryna Sabalenka takes the first set after just short of an hour on Rod Laver Arena.

It's had tension, drama - and it's been unpredictable throughout. We told you it would be a good one!

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There's no stopping that!

Another whopper from the Sabalenka forehand is angled away from Gauff as the Belarusian continues to dominate the tie-break.

Forehand into the corner. Five set points - and surely she won't miss her chance this time around.

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Coco Gauff gets on the board as Sabalenka drops a backhand into the net, but she faces a long road back at 4-1 down.

Pat Cash
Former Wimbledon champion on BBC Radio 5 Sports Extra and BBC Sounds

It's been a little patchy performance from both players but you've got to take your hat off as they continue to go for the shots.

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Sabalenka clenches her fist after hammering in a forehand to take the first point on Coco Gauff's serve for a 2-1 lead.

Another massive hit off the backhand leaves Gauff scrambling, the American tumbling as she stretches but fails to reach it.

Incredible.

It's Coco Gauff's turn to get tight with the first set there for the taking.

The American sends a forehand long and we will have a tie-break to decide a topsy-turvy opener!

Coco Gauff puts herself on track for the set with a backhand for 30-0 but misjudges a forehand at the baseline before netting for 30-30.

There's encouragement from the stands as the crowd try and help the 19-year-old over the line here, but Aryna Sabalenka has other ideas.

Another break point!

Wow! What a change in this match.

From 5-2 down, Coco Gauff hits the front and will now serve for the opening set.

Aryna Sabalenka swings her racquet in frustration after losing her grip on a set she had been dominating with a missed forehand.

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A wild backhand from Sabalenka brings Gauff to deuce but the world number two, who has spoken about her improved ability to control her emotions on court, responds with a winner.

There's no doubt her frustrations will be beginning to grow now though. Gauff has reduced her errors and sticks in a rally long enough to see Sabalenka yield.

Break point!

Twice Aryna Sabalenka has gone a break up in this match, and twice Coco Gauff has pegged her back.

Can she shrug off those setbacks and go and win the first set from here?

Gauff shows she has settled into this now with an athletic overhead for 15-15, before a less-sure Sabalenka misses a forehand for 30-30.

A cry of frustration from Aryna Sabalenka, who misses a volley at the net - and a look at another set point.

Gauff sets up another game point with a big first serve and Sabalenka sends a backhand long to bring us back level in this first set!

Jonathan Jurejko
BBC Sport in Melbourne

We've seen the Gauff facepalm before.

It comes out again as she pushes that backhand wide for a set point.

The US Open champion looks a little glum out there, unable to get herself pumped up.

Gauff produces a timely first serves to get herself out of trouble but she's denied a quick escape when Sabalenka finds the line with another stunning forehand.

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Aryna Sabalenka looks to get back on track with a forehand winner and yet another double fault by Coco Gauff gives her real hope at 30-30.

A bold backhand misses the line! A first set point for Sabalenka...

Pat Cash
Former Wimbledon champion on BBC Radio 5 Sports Extra and BBC Sounds

It [serving] is not something Coco Gauff can't fix. It's about pinpointing what exactly she can do and what she can't do.

When she brings her body weight forward early in the swing, her arms go behind her and her body weight goes forward very early, it's almost like she's collapsing. She's a great athlete and that's what getting her out of it so she's able to spring up and hit some good first serves.

It's just a matter of holding her body weight back a fraction. She can really thump the first serve because she can get her weight forward. It's the second serve that's the issue.