In heat two, Asian record holders Japan (lane five) were bronze medallists in London two years ago and will expect to challenge for the podium again along with Canada (lane two).

GB's Adam Gemili speaking to BBC TV: "I'm actually good. I feel really fresh. It's always an honour to come out with these guys. We were looking for redemption, especially myself after Zharnel, after what happened in the individuals.

Richard Kilty: "We knew the key to this was relaxation. I said to the lads at the start - this is one seamless effort of taking the baton round, and we've done that nicely We're full of confidence, relaxed and ready to go again tomorrow.

Nethaneel Mitchell-Blake: "Right now we've got to rewrite the history books and focus at what's on hand right now."

Michael Johnson
Four-time Olympic gold medallist athlete on BBC TV in Doha

What a mess. Coleman ran a great first leg but Gatlin seemed to already be too far outside on the lane and Coleman couldn't get the baton over to him.

Gillespie in the final lay-off took off way too early. I think it's going to be a toss up whether the US are going to stay in this or not...

As for Jamaica, they finish down in fifth.

Colin Jackson
Two-time world 110m hurdles champion on BBC TV in Doha

Coleman and Gatlin almost barged into Gemili in the next lane.

Andrew Cotter
BBC Sport commentator in Doha

The US handover to Gatlin was very untidy. Messy, to say the least.

That's a season's best time for Great Britain and just a tenth outside the national record.

Nethaneel Mitchell-Blake brings it home for GB by some gap.

It's a new world leading time of 37.56.

Second for Brazil in 37.90. USA third in 38.03 but must wait to see if they progress to the final.

Two messy handovers from the USA slow them right down as Richard Kilty on the third leg for GB flies down the track.

A ferocious start by the USA and a good one too from Adam Gemili.

And they're off.

Andrew Cotter
BBC Sport commentator in Doha

This is a strong heat - the US have gold and silver at one and two with Coleman and Gatlin.

As was the case in the women's event, the top three per heat plus the next two fastest go through to the men's 4x100m final.

Three of the medal favourites all go in this first heat, with defending champions Great Britain (lane six) sandwiched by a powerful-looking USA (lane five) and Jamaica (lane seven).

Time for the men's 4x100m relay heats. A reminder that Great Britain are the reigning world champions in this event.

Two years ago in London, CJ Ujah, Adam Gemili, Danny Talbot and Nethaneel Mitchell-Blake won gold as Usain Bolt pulled up injured in his final race.

I (Katie) was lucky enough to be there that night and oh my word it was good. So let's take another look...

Michael Johnson
Four-time Olympic gold medallist athlete on BBC TV in Doha

Australia's handover, it was a pretty bad one. The outgoing athlete went out too quick and then as she started to slow down, the incoming runner didn't adjust the speed and just ran into her.

I think they could have done something in this because this is not a high-quality relay competition this year.

BBC Sport

GB's Asha Philip speaking to BBC TV: "We trust each other and we knew, in the moment, w can still get it, and we did."

Darryl Neita: "We've practised so hard. We've had an amazing camp coming into this. I feel like we're moving away from the technical side and moving in to the trust.

"We all trust each other. I'm really proud of us and tomorrow is going to be great."

Saj Chowdhury
BBC Sport in Doha

Before KJT's golden reward, there were extraordinary screams from the Italy's relay team after they realised they had qualified for Saturday's final with a national record of 42.90 seconds.

That was as loud as anything I've heard in the Khalifa Stadium during these champs.

Image source, BBC Sport

So this is how the women's 4x100m relay final will look. GB ease through with the second fastest time.

Image source, BBC Sport

KJT looks emotional, she's biting her lip trying to stop the tears.

Then a big, big smile.

High up in the Khalifa International Stadium, Katarina Johnson-Thompson is about to receive her heptathlon gold medal.

What a moment.