Sanya Richards-Ross won the 400m in 2012, but she is not here after she twanged a hamstring earlier in the summer. She has now retired altogether.

Christine Ohuruogu took silver in London but the Brit didn't get through the semis. 

All change. 

British 400m runner Martyn Rooney on Twitter...

Catherine Harrison: I love David Rudisha.

TU: Kipketer you maniac. Messed himself and Bosse up. Rudisha too good, too smart.

An Olympic legend in the making? Possibly.

Allyson Felix goes for gold in the women's 400m final tonight. She has already won four Olympic golds, three in relays and in the 200m in London.

But now she is only concentrating on the longer distance.

She won gold in the worlds in Beijing last year. And is favourite to do it again. 

Image source, Getty Images

Rick: Thought they were rerunning the 400m at the start of the 800m final! Fantastic win by Rudisha to retain his title.  

Alfred Kipketer is only 19 years of age. He has more Olympic finals in the tank. He won't run his next one like that. 

GB's Eilidh Doyle cruised into the semi-finals of the Women's 400m hurdles qualifying in the fifth fastest time 55.46

Lee Fergusson: Another imperious performance from David Rudisha to retain his 800m title at the Olympics.

Brendan Foster
Olympic medallist and BBC athletics commentator on BBC One

David Rudisha looks magnificent in full flow. One of the great sights in athletics, a pleasure to behold.

This man is an impressive athlete, a proud Masai warrior.

Tom Fordyce
Chief sports writer in Rio

Fabulous from David Rudisha, the greatest 800m runner of all time and quite possibly the nicest man in sport too. There will be no-one in athletics who begrudges him his second Olympic title.

Steve Cram
BBC athletics commentator on BBC TV

Alfred Kipketer did everything he could to mess this race up for Rudisha but he wasn’t having it. 

The great David Rudisha. A 1.42 for anybody else would be extraordinary. For him, well it’s almost ordinary. 

But I have no idea what Alfred Kipketer was up to.

David Rudisha has a second Olympic gold medal in his back pocket. Top class. 

The first 200m was run in 23 seconds. Astonishing!

Taoufik Makhloufi of Algeria wins silver, with Clayton Murphy of the US taking bronze.

Alfred Kipketer - who set off at suicide pace - ended up in seventh. 

This is great stuff! Kipketer v Rudisha down the back straight.

The 800m used to be a tactical race. David Rudisha just sprints from the gun. But Alfred Kipketer has gone for it!

Kipketer has run the first lap in 49.23!

Go!

You may remember Nijel Amos running brilliantly to finish second in London. Well, he only finished seventh in his heat and crashed out.

In fact, the two men who finished behind David Rudisha in Beijing in last year's worlds are also not in this final.

Rudisha's biggest threat is likely to come from Alfred Kipketer. 

David Rudisha looks confident. Composed. Controlled.

How close to 1:41 pace can he come?