Del Potro is struggling. His legs have gone. His left leg to be precise. But his right arm is still working alright. The Argentine crunches a wonderful cross-court forehand winner, but that alone is not enough to trouble Murray.

The Briton holds serve to 15 - remarkably, the first hold of this set. Murray edges ahead, Delpo has a rubdown on his troublesome left thigh.

* denotes next server

Olympic legend Carl Lewis - nine Olympic golds, two 100m titles - has been speaking to the BBC before tonight's 100m showdown so we asked him the obvious question.

Would you beat Usain Bolt?

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"Let me put it this way – do you think Jesse Owens would lose to any of us? I ran against the people I ran against. When they put me on the track I had a great career. If you look at someone like a Bob Hayes, who was phenomenal – he went to one Olympics. How can we compare him to my Olympics when I went to four? In reality you can’t compare me to Usain Bolt. You can only compare me to the people I ran with. Even more so, the 100m was my second event – I was a long jumper."  

Not bad for your second event Carl...

Becky Brookes: Soooooo tired but this tennis is enthralling!

Debbie Srao: Let's go Andy Let's go! Let's go Andy Let's go! Let's go Andy Let's go!!! YOU CAN DO THIS!!!

Chris Ingram: Playing Murray must be the worst nightmare. No matter what you throw at him he just keeps coming back. Like the Terminator!

A bronze at least secured in the boxing - and a knockout potentially coming in the tennis.

Juan Martin del Potro looks in real pain as Andy Murray breaks him to level the fourth set at 2-2. 

Neither man can keep their own serve at the moment. Three breaks in a row to start the set and now Juan Martin del Potro shanks his shot long to give Andy Murray three more break points.

Del Potro looks physically wrecked. 

What a performance that was from 23-year-old Joshua Buatsi.

Abdelhafid Benchabla was in his third consecutive Olympic quarter-final. He was his nation's flag-bearer at London. And he's just been given two standing counts against the young Londoner. 

Buatsi wins on unanimous decision, and the worst he'll be going home with is a bronze medal.

Luke Reddy
BBC Sport in Rio

What a point to give Juan Martin del Potro the break. The pair could have hugged at the net. It was like two bulls meeting head on.

The "ole, ole, ole" chant gets a good airing and Del Potro continues to dig his heels in under that 6ft 6in frame. Andy Murray flicked his racket around in anger. He just can't shake this man off.

Is the Olympics making you want to get out of your seat and leap into action? There's so much to choose from - and Get Inspired has an activity guide for everything.  

From Archery to Weightlifting, find out how to take up a new sport near you.

Hands up who remembers the name Robbie Grabarz?

The Briton leapt to high-jump bronze in London and goes in qualifying tonight, along with another GB athlete Chris Baker.

The top 12 men go into the final, while any jump over the height of 2.31m gets you through automatically.

Baker has jumped 2.36m this year, and Grabarz 2.33m.

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A third break in a row! Andy Murray loses his serve once more and it's 2-1 to Del Potro in the fourth set. 

It's not quite as frantic a schedule tonight in the Olympic Stadium - but there's still plenty to get your teeth into.

Including - of course - the biggest name of all in Usain Bolt.

Here's your schedule:

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From 00:30 BST - Men's high jump qualifying (Featuring Chris Baker and Robbie Grabarz)

From 00:35 BST - 00:49 BST - Women's 400m semi-finals (Featuring Christine Ohuruogu and Emily Diamond)

From 00:55 BST - Women's triple jump final

From 01:00 BST - 01:14 BST - Men's 100m semi-finals (Featuring Chijindu Ujah, James Dasaolu, Usain Bolt, Yohan Blake, Justin Gatlin...)

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From 01:30 - 01:41 BST - Women's 1500m semi-finals (Featuring Laura Weightman and Laura Muir)

02:00 BST - Men's 400m final (Featuring Matt Hudson-Smith)

02:25 BST - Men's 100m final

Britain's Josh Buatsi is fighting in the quarter-final of the light-heavyweight competition, and what a fight he is having. 

Abdelhafid Benchabla has taken two standing counts in the first two rounds, and is behind on every card.

If Buatsi wins this fight, he's guaranteed at least a bronze.

Worth watching this again as Jason Kenny wins his fifth Olympic gold medal.

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It all started with Max Whitlock a little over five hours ago. Seven medals down and one more to come...

18:30 (BST): Max Whitlock, gymnastics floor exercise - Gold

19:02: Nick Dempsey, windsurfing - Silver

19:46: Justin Rose, golf - Gold

20:13: Max Whitlock, pommel horse - Gold

20:13: Louis Smith, pommel horse - Silver

21:52: Jason Kenny, cycling sprint - Gold

21:52: Callum Skinner, cycling sprint - Silver

Andy Murray???

Andy Murray loses his serve at the start of the fourth set but then immediately breaks back - that's the third game in a row which the Argentine has been broken. 

Andy Murray is one set away from retaining his Olympic title after winning the third set 6-2. Del Potro's looking tired.

Image source, PA

Will she? Won't she? The big question surrounding Jess Ennis-Hill now is... will she carry on?

And it is not one the two-time Olympic medallist, and two-time world champion, is prepared to fully answer yet.

"I'm at that stage of my career, everyone gets to that point, where I have to decide whether to go on bit longer or call it a day. It's a tough decision," she tells BBC One.

"But I'm proud with what I've achieved I'm content in life and feel I have a great life and balance."

Luke Reddy
BBC Sport in Rio

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Maybe the crowd think that set means this is over? 

We've gone from almost a full house to plenty of space just as the Olympic final gathers pace.

Just behind Andy Murray's seat there's a gent with a tartan hat on and a Scotland scarf. There's no danger of him going anywhere. 

I think his hero has found the gear needed to stretch away and grab history. Hang on a minute though, a Del Potro break in the first game of the fourth. We couldn't be going to five could we?

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The Olympics have been good so far eh?

Like any good festival you were worried at times about the water quality, about the size of the crowd and the late, late nights.

But the supporting acts have shone. Justin Rose nailed the 'legends' slot in the afternoon sun.

You got into some new stuff that you'd not heard of before. Who knows, when it's all over maybe you'll go and see The Trampolinists or The Pommel Horses again.

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But now it's Sunday night. The headliner is about to perform. He's the reason a lot of you are here in the first place. You still talk about that gig he did eight years ago. A game-changer.

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Yeah, there are rumours that his new stuff isn't up to much, he's lost his way a bit. But he knows how to work a crowd.

I have a feeling he'll put out a set of greatest hits and get the whole world watching again.

Over to you Mr Bolt...

Midnight approaches - time for a change of gear. Of focus.