Usain Bolt is high-kicking his way down the warm-up track, cap on backwards, laughing and joking.

He's never looked tense in any race I've ever seen but even by his own standards he looks very chilled. 

Bolt runs in the second semi at 01:07 BST. 

AKA the big one. AKA the fastest men on earth.

Three semi-finals, the top two in each go through automatically.

Jimmy Vicaut of France is the standout name in the first semi (01:00 BST). He's clocked 9.86 this season. He'll need to improve a lot to challenge. 

Michael Johnson
Four-time Olympic gold medallist athlete on BBC Four

It was surprising to see Allyson Felix taking it out really quickly. I am trying to see whether Felix is fatigued from her fast start but it doesn't seem she was. 

I think Felix decided to run like it was a final to make sure she won and Miller decided to run a little bit differently.

Jon Arnold: This match is a case of who falls over first. Del Potro, Murray or the audience

Kurt Sullivan: If Andy can't rap this up before 2:25am, can they take a brief break to allow me to watch the 100m final?

Former champion Christine Ohuruogu is out of the 400m after finishing fifth in her semi-final. The race was won by Phyllis Francis in 50.31

Allyson Felix looks very, very good indeed there to jog to a 49.67 win. She gives the cameras a gleaming smile and then catwalks off the track. Class. Her best time this year. 

Shaunae Miller had to work harder but is through in second.

Can anyone beat Felix in the final?

Luke Reddy
BBC Sport in Rio

Image source, .

Here we have Juan Martin del Potro slumped on the net after another wonderful rally.

Andy Murray looks elated to level at 5-5 in the fourth but even his walk is now a trudge.

Keep going to the well fellas. Eventually only one of you can drink.

What a match.

Allyson Felix, world champion and multiple Olympic medallist, goes next. This is the one title she needs to collect the set. 

Shaunae Miller is also in there. A threat for gold, she's clocked 49.55 this season. 

Emily Diamond on BBC Four: "Overall I am happy. My aim was to make the semi-final and I've done that despite not being at my best. It's amazing to be here and I'm having the time of my life. It's given me confidence that I can compete with these girls and look forward to the World Championships in London next year."

Drama o'clock. Don't you dare slope off to bed. Murray finally nicks that break back after wearing Del Potro down again.

Muzza screams at his box, GB Davis Cup captain Leon Snith almost pops a blood vessel as he roars back. Del Potro looks up to the heavens. He can't believe he has blown that chance.

The first medal of the night in the stadium could come in the women's triple jump final which is about to get under way. 

Defending champion Olga Rypakova of Kazakhstan will be looking to win again. 

Those two break points for Murray? Down the drain. A cute drop-shot from Del Potro saves the first. Then Murray slaps a forehand into the tape. He slaps his head in frustration. Deuce...

An energy-sapping rally leaves Del Potro hanging over the net catching his breath. Rather like you or me after your first sprint on the Sunday morning football pitch after a late Saturday night.

Third break point for Murray. Del Potro recovers his breath to slam down an ace. On we go...

Is that the last time we will see Christine Ohuruogu at a major championship? It sounded like it to me...

Christine Ohuruogu after failing to make the final tells BBC Four:

"I have been stuck in one gear all season. I am disappointed but I am happy I made it this far. These girls are fit and I wasn't able to compete with them today.

"Things don't always go to plan, we are human. It was a tough ask. I haven't broken 51 seconds all season which is unusual but I'm OK. I'm one of the older girls here and have enjoyed some great Olympic moments. I will try not to lose too much sleep over it.

"I think it's one minute to midnight and this girl is about to turn into a pumpkin. I think it's coming to an end, training is getting harder and I'm getting old."

Image source, Reuters

Natasha Hastings and Shericka Jackson qualify for the final easily - and wow, Jackson comes through to win in 49.84! That's a PB. And very quick.

Emily Diamond is out, she finished sixth in 51.49. 

That Del Potro forehand is like a bazooka. Lucky then Murray has some serious defensive armoury at his disposal. He repels a number of missiles to go 15-0 ahead, then targets the tired Del Potro backhand for 30-0. Chance. Two chances in fact. 

Muzza targets the Argentine's backhand again, then switches to the opposite line. Delpo can't cope. Two break points...

GB interest in the second semi too - Emily Diamond is drawn in lane one. 

Michael Johnson
Four-time Olympic gold medallist athlete on BBC Four

This was probably one of the easier semi-finals on paper but it was tough the way it unfolded. 

You normally see Christine Ohuruogu make a move but that never happened. This was just a little bit too quick for her. Her speed was not there.