Oh my word. Where does this come from? Stan Wawrinka racks up three break points thanks to a wonderful backhand which steers wide of Federer's outstretched forehand.

Break point one...disappears as Stan can't control a backhand from the baseline.

Break point two...Federer's second serve has plenty of top-spin, kicking up to force Stan long.

Break point three? He's got it! He rummages into his top drawer, plucking out a forehand winner that leaves Federer - and the Rod Laver Arena - totally stunned.

Wawrinka is serving to take this match into a decider....

Finally some emotion from steely Stan. He screams with frustration as Federer forces deuce. A break now would be critical to his Australian Open survival.

He digs deep, deep enough to take advantage when he lures Federer into a rally which ends with a heavy forehand. And then it is a similar scenario in the next point as Wawrinka holds.

Russell Fuller
BBC tennis correspondent in Melbourne

Wawrinka for the first time is showing signs of fatigue, putting his hands on his knees, as the intensity continues.

Listen to online radio commentary by clicking on the link at the top of the page.

Image source, Getty Images

We've said it before and I'll say it again. Roger Federer's service game is defying the ageing process. He wraps up another sharpish hold quicker than it takes to a stick on the kettle. Two aces leaves Stan thirsty for a break.

This is tense. Like an office where two of your colleagues have just had a blazing row over some missing paperwork. I can't look without clenching my teeth.

Stan's dodgy right knee, heavily wrapped in white strapping, doesn't appear to be bothering him too much. His power keeps Federer at bay, forcing his compatrtiot into a couple of backhand errors on his way to an hold. 

Mickey Wallis: This is truly inspirational stuff from the Swiss duo. What a shame Stan couldn't have started the match at this level!

Oooh. Are nerves getting to Roger? Is he, finally, thinking 'I could win this thing'? A double fault kicking off his service game suggest so. But then like an impish thief, tip toes in, and whips the chance out of his back pocket. He pulls out the next four points to hold.

Image source, EPA

This is more like it. Stan Wawrinka's facial expression has barely changed during the peaks and troughs of this titanic battle. Even when he smashed his racquet over his knee. His poker face remains as he sees out another hold, Federer barely touches his serve.

Akinpelu Kayode: The only thing I can liken Stan Wawrinka's backhand to is Lionel Messi's left foot.

Image source, Rex Features

We have a service hold! For the first time in this fourth set, the server rules. Roger Federer careers into a 40-0 lead with the help of his sixth ace of the match, a Wawrinka smash briefly interrupting his flow. Not for long though.

That first break point goes begging as Federer goes long. Deuce. Can Stan survive? No! An arm wrestle of a baseline rally veers from one side of the table to the other, Stan eventually collapsing when a powerhouse of a forehand drifts the width a chocolate bar past the whitewash.

I hope you've not got any plans this morning. Cancel that meeting. Reschedule that coffee. This is going long. Wawrinka has won six games on the bounce, Federer needs to gatecrash this party.

And he's got a break point at 30-40...

Image source, EPA

BBC Sport

Radio 5 live sports extra is shortly due to switch focus to the first Twenty20 international between England and India in Kanpur.

But, fear not, you can keep listening to the conclusion of this match via the BBC Sport website, either by clicking on the radio link at the top of this page.

It's live now.

Federer to kick off set four. When he's served first he's won the set. Just saying. 

A few groans as he drops the opening point with a stray backhand. Problem? What problem? An ace is what the tennis doctor ordered. Ah, a relapse in the form of a double fault - 15-30. 

Federer scoops out a forehand when he was scrabbling, surviving to force a scampering Wawrinka wide. That saves two break points - but not one.

Stan's confidence is sky-high now. Higher than a member of Little Mix joining a dating app. He floats a dreamy one-handed backhand across the court leaving Federer flummoxed.

Russell Fuller
BBC tennis correspondent on Radio 5 live sports extra

A drop at some point in Federer's intensity wasn't totally surprising but he can reset the button pretty quickly.

We know Wawrinka played a good set but Federer made that quite easy for him.

Did you see this coming? Me neither. Stan Wawrinka comes back from the brink to win the third set in 26 minutes. I suppose Federer had to have a lull at some point, but Stan looked like his head had gone at the end of set two. Remarkable recovery.

Image source, AFP

Is this a warning sign? Federer's forehand is starting to spray wider than an expensive garden sprinkler. He drags one wide for 30-30 before Wawrinka belts a ferocious forehand past his Swiss rival. That's probably bruised the ball like a peach. More power sees out the break point, Federer's wrists not strong enough to cope.

Stan is made of stern stuff. The sort of bloke who refuses to go to the doctors after being downed by the flu for a couple of weeks. Water, vitamin C and paracetamol. That'll sort it.

After battling back with that break, he follows it up with a hold-to-15. Stan in the ascendancy, Federer losing his rhythm. 

Image source, Getty Images

Just a little thought. You remember me telling you Stan whacked his now-dead racquet over his right knee? And you remember me telling you Stan had a medical time-out for treatment on the same knee? Yeah. Wonder if the two are connected...

Anyway, it has not affected him. Three break points for Stan. He attacks a short ball, jumping into a forehand that goes long. Break point number two...Fed clips the tape with a drop-volley, Stan pounces to hammer it home. He's back in it...