How has Madison Keys not won that point? She was in control as the pair rattled off a bit of quick-fire play at the net but she hits a volley wide.

Aryna Sabalenka leads 5-1. She's on a roll.

Aryna Sabalenka gets the first mini-break as Madison Keys falters and double faults.

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Not today, says Madison Keys. An couple of aces help her on her way to a hold against the odds.

To a tie-break we go.

Aryna Sabalenka, who looked down and out of this match just minutes ago, has two set points.

Aryna Sabalenka has finally turned up to this women's singles semi-final. Madison Keys was just one game away from wrapping up the match, but now it's the second seed who looks more likely to seal the second set.

This would be some turnaround if Sabalenka goes on to win.

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Wow. Aryna Sabalenka stops Madison Keys in her tracks.

It seems the American can't hold her nerve when she needs it most and Sabalenka gets the break.

Not so fast, says Aryna Sabalenka. She's not going down without a fight.

She injects more pace into her serve and holds to 15.

Madison Keys will serve for an all-American final clash in New York. Can she hold her nerve?

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All Madison Keys has to do is hold serve twice. Easy peasy.

She magically reaches a Sabalenka return out wide and somehow squeezes it into the court past the Belarusian. Bravo.

Keys, a finalist in 2017, rushes to a 40-0 lead before drawing sighs from the crowd as she pushes the ball into the net.

But it's a round of applause next from the Arthur Ashe crowd as Keys holds.

One game away. You wouldn't bet against her to break Sabalenka's serve.

Madison Keys does well to stay in the rally but she can't find a way past Aryna Sabalenka, who comes to the net and guides a volley out of the reach of the American.

The second seed races to a 40-0 lead and holds to love for the first time.

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Back to deuce we go as Madison Keys goes long with a forehand.

But Aryna Sabalenka follows it up with a replica point before overhitting another.

Keys holds for a 4-2 lead. She's just two games away from a place in another US Open final.

Just as Aryna Sabalenka looks like she is going to turn this match around, she lets Madison Keys straight back in.

The second seed forces deuce but can't deal with Keys' deep forehands to the baseline. Advantage to the American.

Who needs two break points when you can get it in one?

Madison Keys restores her lead with a powerful forehand winner.

Aryna Sabalenka, known for her serve, has held only once so far.

That is brilliant tennis from Madison Keys.

First a majestic forehand winner cross-court, then a backhand down the line which Aryna Sabalenka, despite throwing her racquet at it, can't find the return.

Sabalenka pushes a return into the net and hands the American two break points.

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Aryna Sabalenka gets her first break of the match as she forces Madison Keys into a couple of errors.

Could this be the turning point for the second seed?

Aryna Sabalenka can't make the most of those two break points as Madison Keys pulls her back to deuce.

Nicely done from Aryna Sabalenka as she unleashes a forehand down the line before Madison Keys goes wide to give her opponent two break points.

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Deary me. This is not going to plan for Aryna Sabalenka.

Madison Keys was 40-15 down in that game but she's somehow got the break.

Sabalenka is furious. She's throwing her arms up and down and shouting at her box. The second seed gets a code violation for whacking her racquet off a barrier.

Madison Keys can't get the first couple of backhand returns from Aryna Sabalenka's serves over the net.

However, she pulls Sabalenka to deuce and puts pressure on the Belarusian's serve.

A 114mph ace helps Madison Keys to a 30-15 lead, but Aryna Sabalenka, although she can't stop the hold, looks a lot more menacing now on the American's serve.

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Now, that's more like it. Three aces from Aryna Sabalenka and she finally gets herself on the scoreboard.

She shoves her arms up in the air, but it's more of a sarcastic celebration than one of relief.