Azarenka sprints off for what I presume is a quick bathroom break. Catch you breath y'all... think we might need it.

Image source, Getty Images

And as quickly as it arrived, it disappears as Azarenka fights back.

There is a chunk of luck from a net cord, before a brilliant switch up from the Belarusian ends in a forehand winner down the line.

While it is a shame there aren't 23,000 watching this inside Ashe, it is soooo fascinating to see these two go at it without the fans.

The intensity continues to be amazing. A real insight into the drive you need to be a Grand Slam champion.

Chance for Serena? It's 0-30...

Another roar which can be heard back in Manhattan tells us Williams is edging closer to the hold.

Azarenka finds the tramlines with a crosscourt forehand, then slams a backhand into the net.

"COME ONNNN!" roars Williams. Patrick Mouratoglou just nods and claps his appreciation.

Gigi Salmon
BBC Radio 5 Live Sports Extra tennis commentator

Serena Williams is still swinging, but the footwork has gone. Instead she is planting her feet and hoping for the best.

Williams jogs back out from the chair, trying to work out that ankle and show Azarenka she can still cope physically.

Azarenka has an extra layer of self-assurance/confidence now though, I'd suggest. Strutting and bouncing around that baseline.

Incredible noise inside Ashe... and that's just from the two players. You can hear how much these two are putting into the contest, so much heart and so much quality out there.

Williams is trying to shorten the points now, going for big winners to damage Azarenka's defence.

That comes with its own risk, of course, and the net gets in the way to foil Williams' plan.

Azarenka seals the hold with an 90mph ace out wide.

Image source, Getty

Anyway, the answer to that previous comment is she has got the trainer on and she is back moving.

Williams bounces up and down as she tries to shake off the discomfort.

However, she has bigger things to worry about now... Azarenka takes her second break point and takes charge of the decider.

More tape being wrapped around Williams's ankle. Looks like a plaster cast now. Not sure how she is going to get her trainer on, never mind run and load onto it.

Azarenka closes her eyes and meditates while her opponent receives treatment.

Medical timeout... the trainer is out.

Serena's husband Alexis always look a bit startled, so that doesn't tell us much. Patrick Mouratoglou is up on his feet and looks concerned. That offers us more insight.

Williams's trainer comes off to reveal a surgery's load of strapping around that ankle.

Oh no! This is cruel... Williams stops on the baseline after that point, motionless, just looking down at her left ankle for what seems an eternity.

Something has gone. Ankle roll?

Williams lets out a scream - which sends shivers up my arms watching thousands of miles away - when she puts away a winner for 40-0.

A formality? Not if Azarenka has anything to do with it. She fights back to deuce...

Williams opens the door for an early break at 0-30. Azarenka, in a microcosm of this match, ups the intensity and rattles off four points on the bounce to hold.

A quick sprint to the chair on the way to the other end is a little message to Williams too... "I'm here and I've got plenty of energy."

Image source, Getty Images

Right then, where's your money going?

I'm wondering whether Azarenka can continue that level for another set...

Azarenka hits one, I repeat - one, unforced error in that set.

Twelve winners too. So fresh, so clean.

David Law
BBC Radio 5 Live Sports Extra tennis commentator

This is sport at the very highest level - either player would be a worthy winner.

Image source, Getty Images

Wow. Wow. Wow.

Sometimes that's all you have to say.

Azarenka seals the set in style, a running forehand flashing past Williams and into the canvas.

What a set. What a match. What an evening!

That puts Azarenka 40-15 ahead and, although Williams drags her back to deuce, she gets over the line with an ace.

Williams to serve to stay in the set...

And just to prove the point... these two trade blows in a 21-shot rally which ends when Azarenka finally breaks Williams' resistance with a forehand winner. Even she can't stick strings on that one.

Williams almost drops her racquet as staggers back from the baseline like a WWE wrestler heading back to the ropes after being bodyslammed.

I almost gave that a standing ovation from my dining room table.