Aryna Sabalenka appeared to lose her concentration somewhat in that opening set while Sloane Stephens found a way to deal with the Belarusian's power and was hitting the ball harder and more cleanly.

Sabalenka has a sight of a break point in the opening game of the second set but Stephens recovers well to take the hold.

Jessica Bouzas Maneiro hits long from the net with the court at her mercy and shows clear frustration in response.

It allows Sonay Kartal to establish some daylight and the Briton secures a tie-break with a terrific forehand.

Aryna Sabalenka is an animated figure on the court, fist pumping and yelling "come on!" when she's on a roll and waving her arms around in frustration at the mistakes.

But she calmly walks back to her chair after taking the opening set. She had to dig deep there, unexpectedly so after taking that 4-0 lead.

The defending champion is on track in her opening match.

Image source, Getty Images

Sonay Kartal continues to give very little away but she will be fully aware that this is far from over, so long as she can continue to take her chances.

The Briton makes it to 30-30 as she looks to take advantage of Jessica Bouzas Maneiro's nerves, but the Spaniard finds a backhand winner to edge back in front.

Daniela Hantuchova
Former world number five on BBC Radio 5 Sports Extra

Even though Sloane Stephens has just lost her serve, we have got a match on here.

She is serving much better and hitting the ball harder now.

Another chance comes and goes for Aryna Sabalenka but it's fifth time lucky for the Belarusian as Sloane Stephens hits long and Sabalenka has the break advantage once again.

This set has turned out to be a lot trickier than it initially looked for the defending champion but she's now serving for the set.

Sonay Kartal does brilliantly to stay in a rally and survive a first match point as Jessica Bouzas Maneiro went on the offensive.

From match point saved, to game point, and Kartal is still alive here! Could that prove a significant moment?

Aryna Sabalenka misses the forehand and Sloane Stephens quickly saves the two break points to take us to deuce.

The Belarusian brings up another opportunity, though, as Stephens' wild forehand ends up in the hands of a lucky spectator.

But the American is back at it again as she saves a third break point with a lovely delicate forehand.

Image source, Getty Images

Jonathan Jurejko
BBC Sport in Melbourne

Funny what happens to a player when they’re heading towards the match.

Bouzas Maneiro has been composed throughout most of this contest, but falters at the crucial stage.

The backhand which has been so reliable let her down badly on Kartal’s second break point.

Back in the Brit’s hands…

Sonay Kartal will again attempt to draw level in the second set and apply more pressure on Jessica Bouzas Maneiro, who for such a long period in this match has proven untouchable.

With the pressure back on Kartal to stay in the contest, Bouzas Maneiro relaxes back into things. A confident forehand winner gives her 0-30, and she reaches match point at 30-40 after Kartal nets.

Russell Fuller
Tennis correspondent on BBC Radio 5 Live and BBC Sounds

We are starting to see more anxiety from the champion here.

She is starting to look a little more animated, waving her arms and gesturing towards those in the players box.

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Aryna Sabalenka lets out a big relieved roar as she tries to turn the momentum back towards her and prevent a somewhat unexpected turnaround.

She falters on the two-handed backhand return to give Sloane Stephens a glimmer of hope on serve but produces a powerful smash volley after some back and forth at the net to bring up two break points.

Two break points for Sonay Kartal...

And an overhit backhand from Jessica Bouzas Maneiro, who is clearly getting a bit tight here, gives the Briton another lifeline!

Perhaps the late twists in this match aren't quite over.

A strange choice of shot by Jessica Bouzas Maneiro gives Sonay Kartal 0-30.

Now then, that makes things interesting.

All of a sudden Sloane Stephens has overturned the two breaks by winning three games on the spin.

The defending champion has gone from 4-0 to 4-3 in a matter of minutes.

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Sonay Kartal shows great fight as Jessica Bouzas Maneiro presses on after suffering that first setback, but it might not be enough here.

The Spaniard has a break point after losing serve for the first time and secures an immediate response to move within one game of victory. That's a huge blow for Kartal.

Blimey. Aryna Sabalenka opens her service game with a double fault then Sloane Stephens impresses at the net with an ambitious drop shot which Sabalenka can only pump wide.

The Belarusian lets out a roar as she takes the next point but it's more exasperation as she then makes another mistake to hand Stephens two break points.

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A first hold of the match for Sloane Stephens, who consolidates that break with a much more assured game.

Aryna Sabalenka can only push her return from Stephens' powerful serve into the net.

Jonathan Jurejko
BBC Sport in Melbourne

Image source, BBC Sport

There has been little emotion throughout from Kartal, who does not seem to be a demonstrative person or player.

Even taking her first break point of the match to put the set back on serve does not produce a reaction.

But there was an encouraging cheer from the Brits in the crowd who are willing her to turn this match around.

Sonay Kartal gets her first break point of the match - and takes it!

For the first time, Jessica Bouzas Maneiro's level dips, and Kartal is right there to seize her chance just as her opponent was winding up a dash to the finish line.

It's a lifeline, but now she needs to back it up.