The 26-year-old paid the price for a third-place finish in her semi-final as she was stuck out in lane eight for the medal showdown at the Luzhniki Stadium in Moscow.
Child looked in good shape going through the rounds along with her fellow Briton Perri Shakes-Drayton and the duo were much fancied to challenge the podium positions. She struggled with her stride pattern down the back straight before recovering to finish strongly for fifth place in 54.86sec, while Shakes-Drayton was seventh.
Child admitted afterwards to mixed emotions in a race where her personal best and Scottish record of 54.22 would have been enough to claim bronze. "It feels a bit bittersweet just now," she said. "If you had told me I would finish fifth in the worlds then I would have taken it but that was not my best race.
"I didn't execute it that well and seeing what got medals I think I could have been up there. But it's the third race in four days and sometimes the legs don't have it. So I'm happy to have come fifth and hopefully I can get on the podium next time. I lost my stride on the back straight and felt I was trying to fight back the whole way. However it had been the home straight I struggled with in the heats and semi-finals. If I can just combine the two it would be a better race."
Shakes-Drayton, the reigning European indoor champion over the flat 400m, had qualified for the final as the second fastest. While Hungary's Zuzana Hejnova cruised to the gold medal to cap off a remarkable unbeaten season, Shakes-Drayton faded terribly to finish seventh.
Child was full of sympathy for her team-mate, who had relegated her to silver at the European indoors over 400m earlier this year, after her disappointment. "For anybody who knows Perri, that was not her out there; there was obviously something wrong," she added.
"She wanted to get on that podium more than anything and I think there might have been a problem with her knee but she's a strong cookie. Perri should be proud of what she's done this year, she's had a great year with the indoors and the Diamond League."
Shakes-Drayton herself admitted afterwards that a knee injury before the race had left her hobbling by the time she reached the finish line.
With both athletes likely to be involved in the British women's 4x400m relay squad this weekend, the duo will be hoping her injury will prove not too serious.
"In the race it felt like my knee was wobbling and doing something it should not have been doing," said Shakes-Drayton, who went for an MRI scan straight afterwards. "I am gutted obviously. I was doing so well and feeling so good but I was just not in that race.
"It happened before I clattered into the hurdle," she added. "I was like 'oh god, please hold on' because I was still pushing. I was just losing control I don't know what is wrong with it."
There was better news for two other Scots on Thursday, though, as Jamie Bowie formed part of the men's 4x400m relay team that qualified through to today's final. He ran the third leg along with Conrad Williams, Michael Bingham and Martyn Rooney to finish second behind Jamaica in a season's best time of 3min 00.5sec.
Bowie, somewhat a surprise selection let alone relay starter, was making his world debut in Moscow and admitted afterwards that he had loved every minute of the experience. "It is my first senior international and it is the World Championships as well so I came in not really expecting a run," he said. "I just really wanted to push myself and make sure I didn't let the team down because the rest of them are all experienced guys and it is a big step up and there was a lot of pressure there."
There was success, too, for Laura Muir as she made it through the 800m heats along with Marilyn Okoro on her world debut, finishing third in 2min 00.8sec. "I'm just so shocked," said Muir. "I ran my own race as I knew it was going to go off pretty quick but I've got a strong finish so used that. Down the home straight I felt really good and I just went for it."
Elsewhere, Olympic bronze medallist Robbie Grabarz had to settle for eighth as Ukraine's Bohdan Bondarenko stole the show. Bondarenko jumped a championship record of 2.41m before trying but failing to better Javier Sotomayor's world mark of 2.45m with three failures at 2.46m.
Hannah England just missed out on the podium in fourth in the 1500m final, as Sweden's Abeba Aregawi continued her unbeaten run to take the gold medal.
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