Oh dear, oh dear. Zheng Qinwen calls a shot out and stops playing a point she was very much in.

It's a huge decision - and she has got it horribly wrong.

The chair umpire steps down to inform her, and Aryna Sabalenka is gifts a first set point.

Image source, Getty Images

If Zheng Qinwen is to come through this, she needs to get on top of her serving. She can't rely on her first serve at the moment and Aryna Sabalenka will undoubtedly make her pay.

Sabalenka continues to have joy with the drop shots, too, and Zheng doesn't bother trying to close down this one for 15-15.

Somehow, a seemingly off-balance Sabalenka crouches down to lift a forehand over from the back of the court which catches Zheng off-guard for 30-30.

A rare straightforward hold in this match, as Aryna Sabalenka once again moves within a game of the first set with a hold to 15, quickly shifting the pressure back on to Zheng Qinwen.

A shake of the fist from Aryna Sabalenka as she lands a series of huge groundstrokes at the back of the court to haul Zheng Qinwen back from game point.

But Zheng escapes with a timely ace to get to safety to stay in the first set. On we go.

Greg Rusedski
Former British number one on BBC Radio 5 Live and BBC Sounds

What Zheng wouldn't do for a first serve at this moment...

A poor miss from Zheng Qinwen, who is getting really tight in the final stages of this first set, brings Aryna Sabalenka to deuce. The pressure builds.

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So, from a break up at 4-2, Zheng Qinwen finds herself serving to stay in the set at 5-4 down.

The fact back-to-back double faults played a key part in that momentum swing is a huge mental blow for Zheng after starting this match so well.

Aryna Sabalenka is beginning to get into her groove now and an excellent drop shot catches the eighth seed out for 30-15, before another double fault brings her level. Ouch.

An ace and a double fault for Aryna Sabalenka here too. A lot of all-or-nothing serving going on so far.

A quite ridiculous second serve then scorches the very last millimetre of the line to put Sabalenka back in charge, and the top seed gets clear to lead the first set for the first time since the opening game.

Image source, Getty Images

Greg Rusedski
Former British number one on BBC Radio 5 Live and BBC Sounds

That was very tentative from Zheng. That was playing the name Sabalenka, rather than the opponent in front of you. These small margins make a huge difference, and that's where Sabalenka has matured. She took full advantage of those two double faults.

That's all about Aryna Sabalenka, though!

The world number one makes Zheng Qinwen pay for those double faults with a powerful backhand to bring herself level in the set.

That's going to be tough for Zheng to take, but she needs to dust herself down quickly at 4-4.

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The tension is getting to Zheng Qinwen at the business end of the opening set.

A second double fault in this game give Aryna Sabalenka a first break point.

Aryna Sabalenka's matches are always a great watch because she never hides her emotions. Although, even when she's dominating the scoreboard, she never seems particularly happy with herself.

A double fault from Zheng Qinwen brings Sabalenka to 30-30. The crowd sense the increasing pressure and applaud her before a crucial fifth point.

Abigail Johnson
Commentator on BBC Radio 5 Sports Extra and BBC Sounds

Sabalenka's drop shots haven't even had to be good. The moment Zheng gets to the net, she makes a mis-step. That's something for Sabalenka to target.

Katie Smith
Presenter on BBC Radio 5 Sports Extra and BBC Sounds

There's very different body languages at the moment. Zheng looks very calm and composed, and Sabalenka looks a little irate.

Aryna Sabalenka tries to get herself going with a big shout and a clenched fist as Zheng Qinwen misses for 30-15.

The world number one hasn't been allowed to settle into a rhythm here and she's left frustrated once again as Zheng lands a superb forehand at the back of the court which flies past the Belarusian.

But a miss at the net from Zheng allows Sabalenka to escape - and there remains just the one break of serve in this.

Image source, Getty Images

Greg Rusedski
Former British number one on BBC Radio 5 Live and BBC Sounds

Mentally, that is so tough and so composed from Zheng. She was decisive, knowing exactly where she wanted to put that serve.

A mixed bag for Zheng Qinwen at the start of her latest game, an ace and a double fault combine to leave her 15-30 down on serve.

Boom, another ace! That's followed by an excellent serve out wide, and then a huge backhand down the line to complete the turnaround. She edges a step closer to the first set.

A series of big, booming serves from Aryna Sabalenka swiftly wrap up a hold to love for the world number one in barely 90 seconds.

That's just what she needed after suffering an early setback here.

Image source, Getty Images

Greg Rusedski
Former British number one on BBC Radio 5 Live and BBC Sounds

Zheng has been braver than Sabalenka. Sabalenka has tried to bring her in with the drop shot, which hasn't been effective, and Zheng looks so comfortable at the net.

Can Zheng Qinwen build on that dream start?

She targets Aryna Sabalenka's backhand with a serve for 40-15 and then advances to the net to pop over a closing volley for the hold.

This is super impressive from Zheng so far. For the first time this tournament, Sabalenka looks second best.