A little wobble from Anderson. More plate of a jelly, then Jenga tower though.

A sloppy forehand gives Edmund hope at deuce, but the South African then superbly creates his own angle from the centre of the court, body moving out of the way like he is hula-hooping, to whack a double-handed backhand winner.

Then a forehand winner eases the pressure and heaps it back on to Edmund...

Emily: Obviously rooting for Murray in this match but Del Potro deserves to be ranked in the top 10. What a great player.

The doctor and the physio came out to Del Potro after that last game. He did struggle with a groin problem during his last match.

Back on Murray's serve and there's no let up, Murray sending Del Potro all over the court. A cutesy dink around the net sees him speed to 40-0, before a double fault briefly interrupts his rhythm.

Typical response from Murray - a mutter to himself, a glare up to his box and a "come on!" as he draws the error out of Del Potro.

Image source, Getty Images

Ooh. That's a stonking return from Del Potro, 101mph, and Murray just stands and watches that forehand go firing past him.

That's a bad miss though, the top of Del Potro's racquet catching the ball and the return loops up and out. Just as the Argentine looks like he's getting his nose in front Murray sends a delivery spinning over the net and Del Potro doesn't bother moving.

There goes the big ace from Del Potro, and he'll hold.

Image source, Reuters

Want a good omen Kyle Edmund fans? Luckily we have one up our sleeve.

The British number two has only ever played one five-setter at Roland Garros. He won it.

Admittedly, he has lost the two that he has played since. But let's not focus on the negatives, eh?

Edmund holds to keep his slender advantage.

Del Potro grimaces as his forehand deserts him. This game feels almost sedate now, Murray getting his body behind his serve as Del Potro finds the net again and again.

It's just grown a touch darker over Chatrier. There's some clouds gathering...

Image source, Reuters

While Edmund benefitted from that break, Anderson hasn't. He's lost his rhythm. Well, initially at least.

Off-key backhands put Edmund 30-0 ahead, before a meaty serve leaves the Brit rocking. Edmund tries to go deep in a baseline rally, too deep as it turns out as Anderson levels up at 30-30.

Another thunderbolt of a serve cannot be returned by Edmund, and neither can an ace down the middle that the Yorkshireman can't get a string near.

That wander off court seems to have helped Edmund refocus. He races into a 40-0 lead, a double fault briefly knocking him off stride, before Anderson ensures there is no further drama with a skewed backhand.

Simon Briggs
Daily Telegraph tennis correspondent on BBC Radio 5 live

It's a humid day and, at some point, physically this match is going to turn into a war.

This is good stuff from Murray, who has kept his energy up. Mind you, that's a little sloppy, a straightforward rally shot ending up in the net, and that's deuce.

Ooh, advantage Del Potro! An ordinary return from Murray, but the Briton saves it with another feather touch on a drop shot.

Murray tries the defensive lob again and gets it wrong, before he slaps a forehand out. A hold for Del Potro.

Image source, Getty Images

Right then. After three hours and 18 minutes, we are down to the decider. Only the tennis gods upstairs know how long this could go on for.

Edmund to open up the fifth set...

No fuss. Big-serving Kevin Anderson wraps up the fourth set in just 25 minutes, a complete contrast to the lengthy battles which preceded it.

Not really sure what happened to Kyle Edmund there. A bit of mental fatigue perhaps? Physically he looks fine. He does nip off court for a comfort break - as does Anderson - I'm sure the Brit's bladder didn't affect him too much though...

Ah. That's, er, not so good from Murray with the drop shot, but he recovers quickly to get to 30-15.

Del Potro needs to refocus and do it quickly. His timing is just slightly off, his forehand lacking that extra bit of whip, and Murray batters a floaty drop shot back past him to hold.

* denotes next server

Image source, Getty Images

Murray's used his drop shot to good effect so far. He's recovered quicker than Del Potro, too, and he'll take the early break as Del Potro's forehand goes long.

Image source, Getty Images

The end is nigh for Kyle Edmund in this fourth set. Kevin Anderson nails his second break and is serving to take this into a decider.

Josef Russell: Best tie-break I have ever seen! YES MUZZA!!!!

Simon Gray: Losing that first set tie-break has really hurt Del Potro mentally. He needs to have a rethink and quick.

Piers Newbery
BBC Sport at Roland Garros

It's head-spinning stuff for British fans at the moment with the top two men in action at the same time - cheers from Chatrier ring around the grounds when something big is happening, and there's been a constant din for the last 20 minutes as Murray and Del Potro slug it out.

Somewhere deep in his mind, Kyle Edmund might have been hoping to wrap up this fourth set quickly and pop over the way to Chatrier to watch his mentor Andy Murray.

Kevin Anderson has other ideas. The 6ft 8in South African is keeping Edmund at arm's length after that early break - and with arms as long as his, the Brit is looking like Scrappy Doo as he tries to land a sucker punch.

Apparently, the average set between these two lasts for an hour.

Del Potro looked broken by the finish. He just couldn't believe he'd sent that return wide. Right. Brace yourselves. Here we go again, Del Potro to serve.

Simon Briggs
Daily Telegraph tennis correspondent on BBC Radio 5

The crowd are beside themselves. Their minds are boggling already.

Image source, Getty Images