Jazz Carlin missed out on claiming Great Britain's first medal of the World Championships in Barcelona by only 14 one-hundredths of a second.
Carlin's time of four minutes 4.03 secs in the 400m freestyle final saw the 22-year-old from Wales finish fourth, in a race won by US star Katie Ledecky.
"I'm so gutted, I wanted to be on that podium," Carlin told BBC Sport.
GB's James Guy, 17, was fifth in the men's 400m freestyle final with China's Sun Yan claiming gold.
Carlin was down in eighth position at the halfway stage, but fought her way back in the closing stages, just losing out on the touch to New Zealand's Lauren Boyle, who was third [4.03.89].
Olympic 800m champion Ledecky became only the second woman in history to dip under the four-minute mark in the event - following Italian Federica Pellegrini at the 2009 World Championships - as she finished in 3.59.82, while home favourite Melanie Costa-Schmid [4.02.47] was second.
Carlin missed the London Olympics after suffering glandular fever in the build-up to the British trials but, now back to full fitness, she will also contest the 1500m and 800m events later in the week.
"I fought hard and it's so disappointing to get so close to bronze, but that's what happens in sport and I've got the rest of the week to get that right," said Carlin.
"If someone had told me last week year I'd be fourth in the World Championships I'd have taken it.
"It's my personal best time and I have to take confidence from that."
China's double Olympic champion Yang began his bid to win a historic three World Championship gold medals - in the 400m, 800m and 1500m - with victory over the shorter distance in 3.41.59.
Kosuke Hagino of Japan was second and the United States' Connor Jaeger third.
British teenager Guy's final time of 3.47.96 was a tenth of a second slower than his swim in the morning heats, but overall he was delighted with how his major international debut went.
"Today has been one of the best days of my life, competing here and finishing fifth, I'm just over the moon," he said.
GB will have two swimmers in Monday's 200m individual medley final.
Sophie Allen, 21, secured the fourth quickest time of 2.10.23, while Siobhan-Marie O'Connor, 17, was ninth fastest with 2.11.33 but will swim again after Australian Emily Seebohm opted against competing in the final in order to focus on Monday's 100m backstroke.
There was disappointment in the men's 100m breaststroke where both Michael Jamieson and Ross Murdoch failed to qualify.
Ben Proud, 18, also missed out on the 50m butterfly final with his time of 23.33 putting him 11th in the standings, but he hopes the experience will aid his 50m freestyle campaign later in the week.