Illness has wrecked Gemma Spofforth's hopes of defending her 100m backstroke title at the World Championships in Shanghai.
The 23-year-old failed to reach the semi-finals, finishing 23rd fastest in the heats.
Britain's hopes in the event now lie with Lizzie Simmonds, who made it through to the final.
Elsewhere, Ellen Gandy (fifth) and Jemma Lowe (eighth) missed out on medals in the 100m butterfly.
Hannah Miley was seventh in the 200m individual medley final.
There was disappointment for 10km open water champion Keri-Anne Payne, who failed to make the final of the 1500m freestyle.
Payne, who qualified for the 2012 Olympics with her success in the 10km, was competing in only her third race over 1500m.
She finished as the 12th fastest qualifier in 16:23.11, with the top eight making it through to the final.
Spofforth, who won her world title in Rome in 2009, could not hold back the tears after the chance to defend her title disappeared.
"I've had a bad couple of days," she said. "I was quite sick on Saturday night and didn't eat much. I don't just put it down to that.
"I've had a bit of a bad year and I'm not feeling quite so motivated so hopefully this week will give me a kick up the butt to get me motivated for next year."
In March, Florida-based Spofforth won gold in the 100m backstroke at the National Swimming Championships in Manchester.
She had been determined to perform well at the World Championships after describing 2010 as a "poor year" for failing to claim a title at the Commonwealth Games.
Spofforth wanted to race in Shanghai despite feeling unwell and clocked a time of one minute 1.89 seconds, more than two seconds slower than fastest qualifier Natalie Coughlin's mark of 59.73.
Simmonds qualified in 1:00.38 for the semi-finals and then clocked 59.80 to reach Tuesday's final as seventh fastest.
In the 100m butterfly final, Gandy moved up from eighth at halfway to touch fifth in 57.55 seconds behind American winner Dana Vollmer.
"I am really happy with that swim," said Gandy, 19. "It's the fastest I've gone at this meet so I've got to be pleased."
World 50m backstroke champion Liam Tancock was the seventh fastest qualifier for the 100m final, finishing fifth in his semi-final in a time of 53.60. Frenchman Jeremy Stravius looked impressive, qualifying fastest in 52.76.
"When you come fifth in your heat, you don't expect to make it to the final, although I was really pleased with the time and pleased with what I did from the morning swim," said 26-year-old Tancock.
Ross Davenport and Robbie Renwick both failed to make it into the final of the men's 200m freestyle. Davenport was 10th overall after the semis, while Renwick was 12th.