Salwa Eid Naser is only 18. She has taken a huge chunk off her time, having set a national record of 50.57 in the first round, she's done it again.

Medal chance?

Allyson Felix is such a smooth runner, she floats through the first half of the race. So relaxed, but the field stay with her.

Image source, Reuters

Felix has to work hard in the end and is second as Salwa Eid Naser wins in 50.08 - that's half a second off her PB!

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Salwa Eid Naser(lane 7) set a national record for Bahrain in qualifying in the fastest time of 50.57 seconds. American Allyson Felix (lane 4) also won in a slow heat as she goes for a 10th World Championships gold medal.

10! incredible athlete. She has won gold medals in the 200m, 400m, the 4x100m relay and the 4x400m relay.

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

I don't know what Quanera Hayes, the American champion, was doing there. She was asleep on the backstretch. This is a semi-final you know you have to get out hard, get up to pace. She just didn't look like she was in it at all. At about 200m she looks down but then she looks like there is nothing wrong. Maybe she will get in as a fastest loser.

Hayes ran 50.71. Two fastest losers get a spot from three heats.

Venezuela’s Yulimar Rojas has vaulted into the lead with 14.82m. A long way to go in the final.

The top two fastest losers from the three semi finals go through so Quanera Hayes has a chance. But that was a bizarre race.

Image source, .

Paula Radcliffe gives her take on whether British Athletics have missed medal opportunities over the last 24 hours.

"I think so," she says. "There would have been at least a couple in there yesterday. But I think the 6-8 range was we have two guaranteed for Mo Farah, from the rest we have a whole bunch that everything needs to go right for. There are some that are knocking on the door quietly. Callum Hawkins was disappointed, Sophie Hitchon was disappointed too. That's a good sign as they will go for it next time around."

UK Sport set a target of 6-8 medals. Long way to go yet folks, long way to go.

Shaunae Miller-Uibo of the Bahamas has not lost a race all year - and she comes flying out of the blocks to sail past Quanera Hayes.

Hayes is in trouble at halfway, way back! Only the top two go through and Hayes is third as Miller-Uibo wins yet again. Stephenie Ann McPherson is second.

Image source, Reuters

Sam Bryant
BBC Sport at London Stadium

Anita Wlodarczyk is still not through the mixed zone. The world's media want to talk to her. She's barely made it two steps each time without getting stopped. She seems happy enough to take it all in though - you get used to it when you win as much as she does!

Kelly Adams: Wonder what time athletes get up if they are running at 10pm?

An excellent question. Diet? All sorts of issues.

Quanera Hayes (lane 6) is second fastest on the world list this year and won her heat in 51.43 seconds. Olympic champion Shaunae Miller-Uibo (lane 5) came through qualifying as third fastest with 50.97 seconds and is yet to lose a 400m race in 2017.

Image source, Getty Images

The 2011 400m hurdles World Champion Dai Greene gives his advice to the multitude of men smashing into hurdles through a couple of tweets: "In the 400m hurdles, rhythm and stride pattern is a must have for success at elite level.

"So many guys faltering throughout those races. Only high endurance/strength can help minimise the impact of errors."

She went out big, she limped over the line. But Meghan Beesley did enough to qualify. Just...

Image source, Getty Images

Olympic champion Omar McLeod has been the dominant force in the event this year - the Jamaican has five of the top seven times in the world. America's world record holder Aries Merritt and Olympic silver medallist Orlando Ortega are big threats, while reigning champion Sergey Shubenkov from Russia could win the authorised neutral team's first medal. Britain's Andrew Pozzi went out in the semi-finals.

Next up on track is the women's 400m semi-finals.

It's a tough ask for Zoey Clark to reach the 400m final - but one she's capable of.

The 22-year-old made the semi-final after benefitting from a slower heat to run close to her personal best at 51.88 seconds.

Veteran Allyson Felix is the fastest in the world this year. She's won nine word championship golds already...

Image source, PA

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

He hooked his lead foot under the barrier. These hurdles are weighted, they are not flimsy.

Image source, .

It's not a team event...

Pete Firth: Q for Paula Radcliffe should / could Weightman work with Muir to give GB best chance of gold?

Another mess in the hurdles as Marcio Teles whacks a flight halfway round and he's out, hit it hard.

Abderrahaman Samba of Qatar is smooth though to win it ahead of Yasmani Copello. The big news there is American champion Eric Futch is OUT.