British teenager Jessica Judd was left in tears after failing to qualify for the women's 800m semi-finals at the World Athletics Championships.
The 18-year-old from Canvey Island, who won her event at this year's European Team Championships, finished fifth in Thursday's heat in a time not fast enough for a spot in the next round.
"I expected so much more. I'm so disappointed," she told BBC Sport.
However, team-mates Marilyn Okoro and Laura Muir reached the semis.
Judd added: "I just didn't perform. With two [hundred metres] to go I thought I was going to do it.
"I was clawing them back, but then my legs just went. So maybe that's just some fitness I've lost."
British champion Okoro, 28, was second in her heat in a season-best time of one minute 59.43 seconds to qualify automatically.
Scot Muir, 20, produced an exceptional sprint over the final 100m to take third spot in her race, clocking 2:00.80 - a personal best.
"I'm so shocked," she said. "I thought I'd run my own race and I've got a strong finish, so I could use that.
"A personal best will have to be on the cards again for the next round."
There were mixed fortunes for British champion Anyika Onuora and former world youth champion Jodie Williams in the women's 200m heats.
Both finished fourth in their separate races, but Onuora's time of 23.36 seconds was not good enough to secure a spot in Thursday's semi-finals as a fastest loser, whereas Williams' 23 seconds saw her qualify 17th fastest out of 24.
Britons will also be going for medals later on Thursday, with Olympic bronze medallist Robbie Grabarz in the men's high jump (16:00 BST), Perri Shakes-Drayton and Eilidh Child in the women's 400m hurdles final (17:45) and 2011 Worlds silver medallist Hannah England in the 1500m (18:20).