Jenny Meadows
Former 800m World Championship medallist on BBC Two

Matthew Hudson-Smith chose to stay in the United States, with his coach and his training partners, whereas the rest of the Great British camp have been based out in Miyazaki. They have had 10 days to get used to the conditions, the time change, the heat and the humidity.

I know that our athletes didn't feel good in the conditions for the first five days or so and Hudson-Smith has only been in Japan for maybe four or five days.

Jamaica's Nickisha Pryce wins heat four in 49.91 seconds, followed by Norway's Henriette Jaeger and Roxana Gomez of Cuba, who sets a season best of 50.35.

Great Britain's Yemi Mary John comes home in fifth place with a time of 50.71, which places her second in the rankings of the quickest non-automatic qualifiers. The top six will go through.

Greg Rutherford
Former world and Olympic long jump champion on BBC Two

I think race-planning for the season is something that Matthew Hudson-Smith is going to have to look into.

He opened so early back in April, didn't do that many races after that, and then expected himself to be in top form by September, which was always going to be difficult.

He just looked a little bit rusty, it looked difficult for him out there.

Sydney McLaughlin-Levrone starts her campaign for 400m flat victory by cruising to an easy victory in heat three in 49.41 seconds.

She's over a second quicker than Martina Weil of Chile in second, with Mercy Oketch of Kenya in third.

Dame Jessica Ennis-Hill
Three-time world heptathlon champion on BBC Two

Matthew Hudson-Smith got out well. He looked good and in control but he just lacked energy and that fifth-gear which you need in the final part.

I think it will be mentally quite challenging for him to go away now and come into the next round. He will be thinking about what he needs to do, what went wrong in the latter part of the race and how he can correct things in a really short amount of time.

Harry Poole
BBC Sport in Tokyo

Matthew Hudson-Smith chose not to stop in the mixed zone after his 400m heat, but he has spoken briefly to British Athletics after progressing as a non-automatic qualifier.

"It felt flat, and my hips are a bit iffy," said Olympic and world silver medallist Hudson-Smith.

"I was running and I felt ‘oh no my hips don’t feel good’.

"I got through it, but it hurt man.

"I don’t know, I really don’t know, I literally had nothing. It’s weird."

Madeline Venlogh wins heat two with a new Haitian record of 49.91 seconds, as Natalia Bukowiecka of Poland finishes second and the United States' Aaliyah Butler sneaks into third.

Great Britain's Sam Reardon has been speaking to BBC Sport after his men's 400m heat: "That was tough. The atmosphere was just amazing when I walked out and I have so much family out here so it was very special seeing them as I came into the home straight.

"It was a good race I think, I've obviously had a tough year with lots of injuries - three hamstring tears - so to go out there and still be in with a chance of getting a semi-final spot makes me really happy.

"Lane eight is tough in the 400m. You have to run your own race really but I knew I could go out strong, reserve a bit on the back straight and then work the bend. It was just about trying to get as close as I could to them as possible. I hope it's enough, but you never know."

Reardon's time turned out to be enough to qualify for the semi-finals as the fourth quickest non-automatic qualifier.

Image source, Getty Images

Straight into heat one, then.

Lieke Klaver of the Netherlands takes it with a time of 50.32 seconds.

Sydney McLaughlin-Levrone isn't going to get an easy tilt at the 400m title.

Standing in her way is reigning Olympic and world champion Marileidy Paulino. Last summer, the 28-year-old became the first woman from the Dominican Republic to win an Olympic gold medal in any sport, upgrading her silver from Tokyo with an Olympic record of 48.17. Paulino, who did not run competitively until the age of 19, has only faced McLaughlin-Levrone once, beating her in June 2023.

Instead she has mostly been duelling it out with Salwa Eid Naser, who took silver in Paris last summer. That was the 27-year-old's first medal since she became the youngest ever women’s 400m world champion in 2019, after the Bahrain athlete served a drugs ban that saw her miss the Tokyo Olympics and 2022 World Championships. She beat Paulino in the recent Diamond League Final in Zurich.

Great Britain's Amber Anning became the first British woman to win an individual world indoor sprint title this March, edging out American Alexis Holmes in Nanjing.

She set a national record of 49.29 seconds while finishing fifth at the Paris Olympics but has not been in top form this summer. Her last outing before Tokyo was encouraging though, setting a season’s best of 49.75 for fifth place at the Diamond League Final in Zurich.

She is joined in the field by team-mates Yemi Mary John and Victoria Ohuruogu.

Image source, Getty Images

She's dominated the 400m hurdles, but can Sydney McLaughlin-Levrone now take a world title on the flat?

The 26-year-old has won back-to-back Olympic titles and a 2022 world gold in the hurdles, but the American is looking to add another medal over this distance without any barriers in her way.

McLaughlin-Levrone first targeted the flat event in 2023, but pulled out of the world championships with a knee injury, and returned to hurdling in Paris where she broke her own world record in the final.

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

The quality in this men's 400m event is huge.

There's a huge sigh of relief from us all up in the commentary box to see that all of our British boys are through because the standard is insane.

These heats have been magnificent. We have seen quarter-mile running at it's very best.

Khaleb McCrae of the United States takes heat six, but British eyes will be on those just outside the automatic qualification places.

The fourth place time of Jamaica's Delano Kennedy is slower than that of Matthew Hudson-Smith and Samuel Reardon, so both will join Charlie Dobson in the next round.

Stef Reid
British para-athlete & Paralympic bronze medallist on BBC Two

It's amazing to think that someone who has been so dominant for so long hasn't won this World Championship title yet. It just hasn't gone Valarie Allman's way in either 2022 or 2023.

She is the two-time Olympic champion so you know that she is here to finally take that crown this year.

Image source, Getty Images

The women's discus final is under way.

Can Valarie Allman secure gold after bronze in 2022 and silver in 2023?

The American retained her Olympic title in Paris in dominant fashion with the best four throws of the final. The 30-year-old has eight of the top 10 throws this year - including a remarkable 73.52m in April. No-one had thrown as far since 1989 and it moves Allman to sixth on the world all-time list.

Laulauga Tausaga shocked herself and many others in 2023 by becoming the first American woman to win a discus world title. Her winning effort of 69.49m was four metres further than she’d ever thrown before.

The pair qualified in third and fifth respectively in Saturday's qualifying, with Croatia's Sandra Elkasevic recording the best throw of 66.72m. Elkasevic has two golds in both the Olympics and the World Championships, won in a period between 2012 and 2017. She took bronze in Paris last year and has also won seven consecutive European titles going back to 2010.

Zakithi Nene of South Africa hadn't raced since suffering a grade two hamstring tear on 12 August, but he wins heat five in 44.34 seconds.

Vernon Norwood of the United Sates is second and Hungary's Attila Molnar third.

Qatar's Ammar Ismail Yahia Ibrahim finishes fourth in 44.63, knocking Britain's Matthew Hudson-Smith down to third in the rankings of quickest non-automatic qualifiers. He'll go through unless he drops out of the top six after the final heat.

Image source, Getty Images

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

A very nice run from Sam Reardon. I was watching him in marvel because he has only just come back from injury and he has been thrown into this kind of company.

Reece Holder went out like a man possessed because he knew the quality of the people around him. He thought he would take the race to them all and see what happens. He did what was necessary and ended up with another season best.

Image source, Getty Images

Another Brit home in fourth.

Samuel Reardon gets round in 44.70 seconds in heat four, putting him third of six in the non-automatic qualifying positions with two heats to go.

The 21-year-old finishes behind Christopher Bailey of the United States, Australia's Reece Holder - who had a decent lead going into the final straight but faded - and Zambia's Muzala Samukonga.

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

That third heat moved Matthew Hudson-Smith down one sport in the 'next best' list, so let's just take a deep breath.

These athletes have come to the World Championships and they are raising their games, there's no doubt about it. There's no messing about in these 400m heats.

Wow. Wow. Wow.

Harry Poole
BBC Sport in Tokyo

That's how you get the crowd going!

Japan's Yuki Nakajima runs a national record to progress in the men's 400m and instantly gets the home fans going here.

Matthew Hudson-Smith, meanwhile, lies on the ground for a while as he tries to wrap his head around that performance.

Really strange to see the Olympic and world silver medallist struggle like that at this stage.