Three-quarters of Centre Court is in shade now. Nadal just dips into sunlight when he runs to retrieve the ball on his backhand side behind the Royal Box.

The effort of both players in this match is clear. They're groaning on every ball.

Del Potro shows great reactions at the net to beat Nadal with a volley and holds.

Image source, Reuters

Del Potro's team have thrown something down from the players' box to a ball boy which is then passed to the Argentine. DelPo gives a thumbs up but stuffs it straight in his bag. Probably a snack for later.

He's still in this match after an excellent comeback in the second-set tie-break. Here's how he clinched it...

Meanwhile, Rafael Nadal has just breezed through a service game but level it up. Who are you backing?

The Del Potro serve is on form.

He hammers an ace down the middle and the ball almost screams in pain, fluff everywhere.

Nadal gets a grip of the next and forces the fifth seed long, but DelPo follows up with an ace to hold.

The other quarter-final between Milos Raonic and John Isner over on Court One has gone to a tie-break in the first set.

And it's Raonic who takes it.

Watch that match here.

Oooh, a wayward forehand from Rafael Nadal that skips into the tramlines.

He gets Del Potro on the run off the next with some rapid groundstrokes and smashes an overhead down in the Argentine's direction - Del Potro almost somehow gets it back on the run, but just wraps the tape.

A Nadal slower serve flummoxes the fifth seed and it remains on serve.

Image source, Reuters

Roger Federer after his loss to Kevin Anderson: "I couldn’t feel very well off the baseline. The one-two punch wasn’t working at all today. I don’t know if it was the breeze or just a bad day from my side. I didn’t feel exactly 100% and that had nothing to do with my opponent, it is just one of those days where you hope to get by somehow, I almost could have and should have. Once you are in the rally it is hard to get him moving because he hits hard and strong.

"I felt good. Sure it is disappointing losing the next two sets after winning the first two and having match point but I’ve been there and I know what kind of energy I need to bring in the fifth and I was able to bring that.

"I didn’t feel mental fatigue. Now I feel horribly fatigued and just awful. It’s terrible. That’s how it goes, credit to him.”

Juan Martin del Potro scuttles into a short ball, sizes it up but then sticks it straight into the tape. Centre Court lets out a collective groan.

The Argentine works Rafael Nadal around with some punchy forehands before sizzling down a winner, and backs it up with an ace out wide.

Another solid hold.

That's better from Rafa, soft hands to slip a slice just over the tape and leave Juan Martin del Potro too much ground to make up.

He sprints to a love-hold.

Can't go round him? Can't go over him? Rafael Nadal tries to go through the big Argentine, but Del Potro manages to eke out a volley from his midriff and takes the point.

Nadal responds with a flick of the wrist to get on the board, then tickles a deft drop shot over the net - but DelPo's there, and he pops it beyond the world number one to hold.

Image source, Getty Images

Right then, Rafael Nadal will think he should have been staring at a two-set lead after having three set points in the tie-break.

All wasted...

Momentum with Juan Martin del Potro?

He saunters into the net to slap away a grunting Nadal with a neat volley.

Centre Court goes wild.

Juan Martin del Potro levels it up!

Set point saved with the big serve by Del Potro.

And another gives the Argentine set point for the first time in the match...

Nadal gets lucky. He gets a flick off the tape. Del Potro roars in frustration.

Another set point for Rafa...

Double fault from Rafa Nadal! On set point!

The whole of Centre Court gasps as one. Del Potro is still in it.

Image source, Reuters

Del Potro saves the two set points on his own serve but now Rafa has one on his own racquet...

Juan Martin del Potro has his head in his hand after putting a backhand into the net. Now he slaps his forehead. He's furious.

Centre Court tries to get behind him - "come on DelPo" they shout - but it's looking bleak.

Three set points to Nadal...

You'd choose Rafa to win a match for your life wouldn't you?

He's in control at the changeover despite another thunder serve from DelPo.

Rafa Nadal is saved by the technology and only just. His serve is called out but by a matter of millimetres Hawk-Eye overrules.

Del Potro responds with a serve that is through Nadal before he completes his swing but Nadal gets the mini-break a point later.

Image source, BBC Sport

Andrew Castle
BBC Sport tennis commentator

Kevin Anderson is a very well organised player and managed to turn his mind off all the pressure when he was serving for the match.

The implications were huge for him - he will probably be favourite to win the semi-final and make the final now.

The serve returns at the perfect time for Juan Martin del Potro. He digs deep and gets through.

It's tie-break time.