Katinka Hosszu leads at the halfway stage. Gold, as expected, looks to be a stretch for Siobhan-Marie O'Connor, but she's in the mix for silver...

Siobhan-Marie O'Connor going stroke for stroke with Katinka Hosszu after the butterfly...

And go! 

I've counted them, there's 20.

Might need to add another one before the night's out, though, Mr Walker.

1 Sydney Pickrem (Can)

2 Melanie Margalis (USA) 

3 Maya DiRado (USA)

4 Siobhan-Marie O'Connor (GBR)

5 Katinka Hosszu (Hun)

6 Ye Shiwen (Chn)

7 Alicia Coutts (Aus) 

8 Viktoriya Andreeva (Rus)  

Tom Fordyce
Chief sports writer in Rio

Four years ago Siobhan-Marie O'Connor was Britain's youngest swimmer at the London Olympics, a wide-eyed 16-year-old chasing experience as much as a medal. Not this time. As the fastest qualifier from the semi-finals she has the queen slot in lane four - but she will have to come past the fearsome Katinka Hosszu in five to win GB's second swimming gold of these Games. Is the Iron Lady for turning? We shall see...

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

Siobhan-Marie O'Connor will be feeling sick before the race. You feel like you want to throw up, it's not the nicest experience. I used to get so nervous.

I think we'll see Hosszu break the world record but hopefully Siobhan can get close enough to her to put pressure on her.

Heading into the closing stages of today's swimming and now it's time for the big one as far as Team GB is concerned. Siobhan-Marie O'Connor is up in the women's 200m medley and has a real chance of a medal.

Awwwww! 

It's all getting a bit emotional in the Olympics Aquatics Stadium as Michael Phelps goes up to hug his baby son, Boomber, and they are swiftly surrounded by a horde of photographers. It's all too much for Rebecca Adlington. The tears are flowing.

Adrian Moorhouse
1988 Olympic 100m breaststroke champion on BBC TV

That gold means a lot to Phelps, to win gold in the event he started out in back in 2000 in Sydney.

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

He's a great character to have on the team and very funny as well. What a fantastic swim. It will be such a close final tomorrow night.

GB's Andrew Willis after qualifying for the men's 200m breaststroke final: "I'm dead chuffed with that. I felt good. I tried to pick it up on the back end. A PB and I'm chuffed with that. It's all to play for now.

"I've been in this position so many times. I'm not going to take anything for granted. I'll try and stick to what I did tonight and hopefully the rest will mess around a bit and l sneak through."

Michael Phelps right now is doing what he has done 19 times before - climbing the top step of the podium at the medal ceremony. 

The announcer is drowned out as he starts to introduce Phelps to the crowd, who roar their appreciation for him. Legend.

Image source, AP

Adrian Moorhouse
1988 Olympic 100m breaststroke champion on BBC TV

That was a personal best time for Willis. He's delivered and should be really pleased. He held his technique, was very strong and finished well. He's been consistent throughout his career since 2012.

Excellent stuff from Andrew Willis. He holds his technique before powering through in the final 50m to finish first in 2:07.73 and he's into the final.

Craig Benson won't be joining him, though. He finished seventh.

First to walk out to his lane is Craig Benson with a massive, massive coat on. 

He won't feel the benefit when he goes outside.

Strong stuff from Ippei Watanabe in the first breaststroke semi-final, setting a new Olympic record of 2:07.22. He was so close to a new world record too.

Next up, Andrew Willis and Craig Benson...

Funny to think that TV presenter Ryan Seacrest has nearly 15 million followers on Twitter, but Olympic swimming legend Michael Phelps yet to hit the 2m mark. We think Michael will probably get over it.

GOAT = Greatest of all time. But you knew, that right?