That was a mammoth serve. It nearly ripped the racquet out of Stephens' hand.

That's the impetus Venus needs to close out this game.

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It's been an error-strewn third set from Venus. She was catching the line with some thumping strokes in the second, now they're a tad too long.

The two-time US Open winner scrambles back with a big serve and an even bigger backhand to floor the scampering Stephens. But another wayward Venus return brings us to deuce.

Stephens has Williams pedaling side to side like a kid in PE doing the Bleep Test. She passes the first few levels with flying colours, but the ninth seed can't keep up as Stephens increases the tempo and Williams punches a wayward backhand beyond the tramlines.

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Russell Fuller
BBC tennis correspondent at Flushing Meadows

You earn your own luck in life and that was a brilliant bit of scrambling defence from Sloane Stephens to force Venus Williams into one more shot and save the break.

Someone's turned the volume up a notch inside Ashe. Williams is on the run, but still manages to manufacture a sliced lob... it's long.

Stephens somehow reaches a huge Williams forehand to keep herself alive in this game, and Venus looks bemused.

Sloane Stephens is reaching deep into her armoury to fend of Venus Williams now. The world number nine sliding through her shot selections at just as quick a pace to try and outplay her younger opponent.

Stephens opts for the forehand smash. And shanks it. Two break points for Williams.

Image source, Reuters

Jeff Tarango
Former professional tennis player on 5 live sports extra at Flushing Meadows

That break was just what Sloane Stephens needed to stop the momentum of Venus. She seems to have mentally reset.

She takes it! Williams encroaches towards the net and Stephens fires one right at her. The veteran can't adjust quick enough and it's Stephens who makes the first move on this third-set scoreboard.

That's long from Venus Williams. And a break point chance in the third set for Sloane Stephens.

Two sets of stark contrast. The first dominated by Sloane Stephens, the second obliterated by Venus Williams.

It's the latter to serve from the start of the decider. The screeches are getting louder, the strings hotter.

And with that, a glorious crosscourt winner from Williams as she strides into the tramlines to retrieve a Stephens return and hammers it back with interest. 30-30.

Jeff Tarango
Former professional tennis player on 5 live sports extra at Flushing Meadows

You could never have predicted this match. Just as you could never have predicted the last eight of this draw.

If you managed to pick the quarter-final line-up in this ladies draw, you would deserve every one of the millions that Vegas would give you.

I think that we will see Sloane Stephens step up to the mark and come back at Venus.

And Venus Williams snaffles it up. Sloane Stephens looks to throw everything into a crosscourt forehand, but it's halted by the net.

It's all square in this semi-final.

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Whatever Sloane Stephens does, Venus Williams seems to be equal to.

The world number 83 is finding the angles, the ball is just coming back quicker and more acutely. It's a set point for Williams.

The Arthur Ashe Stadium is filling up now. A few pockets of blue still dotted around. Must be a queue for the drinks stall.

Venus Williams comes steaming towards a low Stephens forehand and wallops it back with interest before it can dip over the net.

The younger American is a least sucking her older compatriot into longer rallies now, and that's roused the crowd.

Williams goes short, Stephens races in and goes shorter... but Venus reaches out a limb to force a winner. Stephens can only stand, admire and clap the 37-year-old. Deuce.

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Jeff Tarango
Former professional tennis player on 5 live sports extra at Flushing Meadows

It is called a 'Euro-tank'.

You let the set go, keep your powder dry and give it everything in the next.

European players live for it.

Maybe that is what Stephens is up to.

Sloane Stephens needs to rediscover the composure she showed in the first set.

She's visibly frustrated in this second as another forehand drifts long. A sigh from the 24-year-old and she engages Williams in a rally on the next point, but can only take a chunk out of the net with a reaching backhand on the run.

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There. We. Go.

Return deep, dash to the net, slice volley winner. That's textbook from Venus Williams. And she goes four up.

A swivel of the racquet, a slow second serve, and that's punished by Williams. Spinning its way beyond the flat-footed Stephens.

Now Venus is cooking. The signature dish a fiery forehand down the line. Two break points.

Image source, Getty Images

Jeff Tarango
Former professional tennis player on 5 live sports extra at Flushing Meadows

I think the best analogy is that you go to the Bugatti shop, you don't just jump in one of those machines and bomb off down the street at 80 mph.

You have to work out how these things work, whether your insurance covers it and finally, belatedly, you get it going and suddenly you are flying.

That is what it has been like for Williams in this second set.

Now Venus is looking at home. So she should, the two-time US Open champ played her first match on this court when Sloane Stephens was just three years old.

All that experience is coming to the fore now. Consistent length, waiting for Stephens to misfire. She cruises to a hold to go 3-0 up in this second set.