As expected Katinka Hosszu has gone from the gun. She wants gold and a world record. Flying start. 

Spanish swimmer - in lane five - Mireia Belmonte Garcia is allergic to chlorine! That's awkward...

GB's Hannah Miley was fifth at London, sixth in Beijing.

Nobody wants a fourth so it must be time for a medal, right?

Image source, Rex Features

James Guy, speaking to BBC Radio 5 live after finishing fourth in the 400m freestyle: "It was trying to go out hard and see how I could go on. Sun and Mack are distance boys so it was going to be hard. I feel like I am in a good place, you move on. It is what it was. 

"I'll go back now and chill, watch the Inbetweeners and have some dinner and enjoy tomorrow.

"I am at the Games, I am 20-years-old. You move on."

Louise: One more chance for a team GB medal on the first day hopefully happens  

Linda Savage: 2am alarm call worked, up and ready to watch the swimming - just trying not to cheer loud enough to wake the neighbourhood.

1 Sakiko Shimizu Japan

2 Aimee Willmott Great Britain

3 Maya DiRado United States

4 Katinka Hosszú Hungary

5 Mireia Belmonte Garcia Spain

6 Hannah Miley Great Britain

7 Elizabeth Beisel United States

8 Emily Overholt Canada

Image source, Rex Features

Up next is the women's 400m IM final. Hungarian Katinka Hosszu is tipped to break a world record in this one. No pressure...

Image source, Getty Images

James Guy, speaking to BBC One: In the last 100m or so I could see Sun and Mack going past and I just thought oh no. My family flew all these miles to see me swim but my first 200m was pretty comfortable so it is good times for [the 200m] tomorrow.

Image source, Getty Images

Helen Lewis: So happy @_mackhorton, external won that gold medal. Also very brave swimming from James Guy with the 200 free tmrw.

Glen Naylor: Gutsy from James Guy. Bodes well for 200m free. Mack Horton beating alleged drug cheat Sun Yang for gold popular result

Surely an Olympic final is exactly the time to take a risk? If you're not a favourite? I'm all for it James Guy. Better than going out steady and ending up sixth anyway...

Rebecca Adlington
Double Olympic gold-medal winning swimmer on BBC One

I personally think it was very brave of James Guy, but for me not the right tactics for the Olympics. Now is not the time to take a risk in my eyes. He was great to 200m, that is his main event and I am really excited for that tomorrow.

Steve Parry
Olympic bronze medal swimmer on 5 live

There's only a few moments you get in sport as exciting as that. I'm covered in goosebumps. Mack Horton took it to Sun Yang and what a fantastic finish. They are not shaking hands, Mack is swimming past Sun Yang but you don't have to be friends in sport.

Two - count them - TWO Brits in the women's 400m individual medley, which is set for a clubbers' exit time of 02:49 BST.

Aimee Willmott and Hannah Miley go for the medals, with Hungarian Katinka Hosszu streets ahead of the field.

She qualified at least four seconds ahead of the rest. Has she gone too hard, too soon? Or is she nailed on for a first Olympic gold?

Image source, AP

Kosuke Hagino won the men's 400m individual medley to give Japan the first swimming gold medal of the Rio Games.

Here's how he did it.

James Guy STILL leads but he's tiring now. Surely he can't hang on from here.

James Guy still leading. This is some swim - still 150m to go though...

Steve Parry
Olympic bronze medal swimmer on 5 live

James Guy is leading. He has obviously changed his tactics and has thought he has to take this to the rest of the field. My worry is, is he going out too soon?