Image source, Reuters

What a winner from Istomin! He was scrambling across court to his backhand side after a well-placed Djokovic approach and fires one just inside the opposite tramline, before shooting the Serb a telling glare.

The world number two then pulls it back to 15-30.

Was that a little fist pump from Djokovic? I think so, just a little one though - he'll want to expend as little energy as possible after four hours and 27 minutes of this second-round clash.

But that winner - and mini fist pump - comes in vain. Istomin holds serve again with a smash to finish.

Istomin checks his glasses are in place, adjusts his sweat-soaked shirt and then pushes Djokovic around before darting to the net.

But the Uzbek is a little lethargic in trying to pick that one up over the net, his half volley drops short and Djokovic takes the point.

That doesn't put Istomin off though, next rally and he's up to the net quickly again to take it on the full this time and place one across court to make it 15-15.

Djokovic has Istomin on the run now, just a blur of lime green darting across court thanks to the world number 117's brightly coloured sunglasses and head band.

He can't make any indent on the Djokovic service game though, and the Serb wraps that one with little fuss. 

Djokovic pulls it back to deuce. The times he's managed to contain Istomin's first serve the world number two has been able to control the point.

But Istomin hammers one down that Djokovic can only prod an outstretched racquet at and then takes the game. All square again.

Mind you, Denis Istomin isn't short of a big serve either. He has Djokovic scrambling and still looks strong at the start of this fifth set.

A well-placed passing backhand gains the Serb a point, and after engaging Istomin in a rally he takes another as the world number 117 finds the top of the net.

So Novak Djokovic hasn't lost before the Australian Open fourth round since 2006...

He's stormed through his first service game in the final set, clinical from the world number two.

Will that ease the Serb's nerves a little?

Image source, Getty Images

Well, it's not going smoothly for defending champion Novak Djokovic.

The six-time Australian Open winner has been taken to a fifth set by world number 117 Denis Istomin.

Djokovic lost the first set 7-6 (10-8), before taking the next two 7-5 6-2. But the Uzbekistan's Istomin has just won a tie-break in the fourth and that's where we'll pick this up...

Image source, Getty Images

With Novak Djokovic a set down against Denis Istomin, we are going to leave this live text right here.

You can keep up to date with the live scores page though as the world number two tries to dig himself out of that medium-sized hole.

G'day.

Five match points have come and gone for Heather Watson and now, after double-faulting at 0-40 down, the Briton has to break back to avoid defeat.

Watson is turned away from the inn once again as Jennifer Brady finds whopping first serves twice from 15-40 down.

Novak Djokovic has lost the first set against Denis Istomin over on Rod Laver Arena, going down 10-8 in the tie-break in one hour and 25 minutes.

Translate that to five sets and you would have a seven-hour epic.

Heather Watson holds up her end.

The two are like prizefighters slinging everything into the final round.

Can someone finally land the knockout blow?

A 12th ace of the contest from Jennifer Brady makes light of that pressure.

Over to you Heather.

We are just shy of two and half hours,

Heather Watson served first in this decider and so has the scoreboard tag-teaming alongside her against Jennifer Brady.

Brady will serve to stay in the tournament once more.

Another match point faced down by Jennifer Brady and on we go.

No final-set tie-break . They save that for the States.

Watson pings an ace down the middle to finally snuff out the danger.

That was gruelling.