Jill Craybas
Former world number 39 on BBC Radio 5 live sports extra

Wawrinka has responded beautifully after losing that first set.

Image source, AP

Nishikori has an opening again on his opponent's serve.

0-30 and he goes for a big backhand return but can't control it.

The point of the match follows with Nishikori scampering to retrieve on the baseline again and again but Wawrinka wins it with a volley.

He sends a forehand long moments later though and Nishikori has break point.

The serve saves Wawrinka not once but twice as another chance to break goes by for Nishikori who blasts a ball into the crowd in disgust.

Two more big serves and Wawrinka has found his way out of another difficult service game.

This is great stuff.

The crowd are really into this one now. 

Nishikori gives Wawrinka the run around as he comes to the net yet again. I don't remember him moving forward this often in previous tournaments. 

The influence of his coach Michael Chang, a US Open finalist in 1996, perhaps? 

A sliced ace out wide and we're level again in this set.

David Law
Radio 5 live sports extra commentator

It is a fascinating match and anyone's guess who is going to win this second set.

Image source, AFP

Oh no what has Wawrinka done here? 

He's furious with himself. An error off the forehand side before a double fault and now he's facing three break points.

First one saved. A big serve does the job.

Second saved as Nishikori misses with an ambitious forehand.

We have drama. A late line call as Nishikori pushes the ball towards the baseline. He challenges but the line judge was right. Deuce.

An 18 shot rally follows and Nishikori wins it with a fierce backhand down the line. Huge cry of 'come on' as the Swiss saves the break point and hits a forehand winner for game point.

Big backhand down the line to take the game. Classic Stan Wawrinka. 

A fist pump, he knows that was a big hold of serve.

Gael Monfils has had to field questions at his news conference questioning his desire to win his semi-final with Novak Djokovic.

The world number one won in four sets, after Monfils played at walking pace at the end of the first set and start of the second.

Wawrinka can sense his man is struggling.

Nishikori puts a volley into the net and then double faults and we're at 30-30.

Good stuff from the Japanese player though as he holds serve with a vicious smash at the net. He's halted the momentum that was heading Wawrinka's way.

Jill Craybas
Former world number 39 on BBC Radio 5 live sports extra

My prediction before the match started was that it would go five sets. They both move very well, don't give much away and I think we are in here for the long haul.

This suddenly has a different feel about it. Wawrinka was struggling but now the ball is flying off his racquet.

He wins the game with a backhand winner and is ahead in the set for the first time.

A few nervous looks upwards, rain is forecast but we're good to carry on for now. Wawrinka definitely won't want a break. He's found his form in this match.

Now then. What can Wawrinka make of this, he's won two Grand Slams don't forget.

A double fault and a forehand error from Nishikori takes us to 15-30.

He sends a forehand into the tramlines two points later and Wawrinka has a break point.

Brilliant hitting from the Swiss player and he takes it. Wawrinka's right back in this second set. The match is really hotting up now.

Jill Craybas
Former world number 39 on BBC Radio 5 live sports extra

Nishikori is playing incredible tennis. He has been in this position before and has been in the final here. He has looked the sharpest of all of the semi-finalists today.

Nishikori likes it when you hit hard at him and he is so fast around the court so it does not intimidate him. Wawrinka needs to mix it up a bit more and get Nishikori out of his rhythm because Nishikori is loving the pace at the moment.

Image source, AFP

Better news for Wawrinka fans in that game.

A couple of errors on the backhand from his opponent and he holds much more comfortably than previously.

These two are playing beneath the night's sky in New York now. There is something special about night matches at the US Open isn't there?

David Law
Radio 5 live sports extra commentator

Wawrinka is having to go for broke on his ground strokes to keep Nishikori away.

Image source, Getty Images

This is pretty one-sided stuff.

Nishikori is running his opponent ragged 

That's more of the Wawrinka we were expecting as he crunches his first backhand winner of the match.

Brilliant from Nishikori though. Every ball out of the middle of the racquet and he plays a brilliant final point to take a two-nil lead in this second set.

The stats make interesting reading after that set.

Wawrinka has hit more aces and more winners but it is Nishikori who is ahead, he hit just two unforced errors.

Accurate and clinical from the Japanese player who didn't really give Wawrinka a sniff.

And here we go again. A jump-smash at the net sets up three break points for Nishikori.

He takes it on the second attempt with a big forehand return winner, Wawrinka gives a worried look towards his box. 

Image source, AP

First hint of pressure on the Nishikori serve. 

A double fault and we're at 30-30 but again Nishikori is in at the net to volley away the winner to give himself set point.

A big serve out wide and he takes the set. Very impressive stuff he's continued right where he finished in the quarter-final.

Jill Craybas
Former world number 39 on BBC Radio 5 live sports extra

Kei Nishikori looks a bit tired and on the changeover he was definitely putting the ice on the back of his neck trying to cool himself down. It is still so humid.

Image source, EPA

Wawrinka is really being forced to work for this but a big serve at 40-15 and he holds.

He'd be expecting to win more points than his opponent on first serve and has hit five aces to Nishikori's zero so far.

But the Japanese number one has won 92 per cent of points behind his first serve, more of that and he'll be one set to the good.

Beautiful touch from Nishikori with a drop shot. Stan doesn't even move for it.

Showboat stuff to win the game with a top spin forehand and a backhand volley right into the corner.

He's one game away from the first set after just over 20 minutes.