Williams has made just 43% on her first serve, and only won 27% of her second serves so far.

Jonathan Jurejko
BBC Sport at Flushing Meadows

"You're a superstar, yes, that's what you are, you know..."

Madonna blares out almost as though the DJ is paying tribute to Bianca Andreescu.

Bianca Andreescu, at just 19 years old, is just two games away from her first Grand Slam title.

David Law
BBC Radio 5 Live tennis commentator at Flushing Meadows

She's totally wrong-footed. I don't know what Serena's done there. She is discombobulated right now.

Image source, AFP

More Williams errors allow Andreescu to fly to 40-0. A big winner from Williams stops her from holding to love.

Oh my word. Williams loses her way going for a backhand, stopping herself mid-shot before pretty much passing it back to Andreescu to slam home for the hold.

Williams throws her hands up, she just doesn't know what to do.

David Law
BBC Radio 5 Live tennis commentator at Flushing Meadows

Andreescu is just too good! Watching these two players going toe-to-toe and seeing someone overpowering Serena the way she does, you almost have to rub your eyes to believe it's happening.

Williams is broken again, skewing a forehand well out of the court.

Crikey.

David Law
BBC Radio 5 Live tennis commentator at Flushing Meadows

Williams just cannot keep Andreescu away on her own serve.

At 30-30, Williams sends a sensational ace down the T and roars to the skies. Andreescu challenges but it doesn't go her way.

Andreescu levels at deuce and Williams is laughing. I think at herself...

She slams into the net and it's break point for Andreescu.

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Williams has made just 46% of first serves so far.

Despite that break, she looks seriously dejected and somewhat emotional.

Image source, Getty

David Law
BBC Radio 5 Live tennis commentator at Flushing Meadows

Serena gets the luck! People are on their feet in front of us, punching the air.

This time she takes it! Albeit with a little help from the net cord.

She holds her racquet up in apology to Andreescu but my oh my did she need that. Is this a turning point?

This time she takes it. Or does she? Andreescu goes long and Williams throws her arms up, but the Canadian challenges. Oh my word, Hawk Eye shows it was IN.

Back to deuce. But another winner from Williams and it's a fourth break point.

And the exact same thing happens again, a backhand into the net to bring us back to deuce. Where is Williams' A-game? I'm not even sure she's brought her B-game.

But she battles to another break point. Third time lucky?

She scuppers it, a backhand into the net. The camera pans to sister Venus in her players' box, she's sat looking at the floor shaking her head...

Another chance though, as a Williams winner brings up another break point.

The opening points are exchanged on Andreescu's second service game of this set, before it's the teenager's turn to double fault on serve.

Break point Williams.

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You could hear a pin drop on Arthur Ashe. The crowd are stunned into silence.

David Law
BBC Radio 5 Live tennis commentator at Flushing Meadows

Williams is down. She is struggling suddenly.

Image source, AFP