Lithuania's Ruta Meilutyte broke the 100m breaststroke world record in the World Championship semi-finals.
The Plymouth-based 16-year-old clocked one minute 04.30 seconds in Barcelona, 0.15 secs faster than the previous best time set by Jessica Hardy in 2009.
Earlier, Meilutyte had recorded 1:04.52 in qualifying, the second-fastest time in history and a new European record.
In the 200m individual medley final, Sophie Allen was seventh and fellow Briton Siobhan-Marie O'Connor eighth.
London 2012 Olympic champion Meilutyte will seek to win Lithuania's first ever World Championships gold medal in Tuesday's final.
"I'll be have a relaxing day tomorrow and will save all my energy for the final," she said.
"My biggest aim is accomplished now and I'll give it my best shot in the final. Obviously, a gold medal would be a cherry on top of the ice cream."
Her coach Jon Rudd said: "It feels a bit surreal, but now I remember the feeling of the Olympic gold medal because it's kind of the same.
"The plan was to go for the world record tonight because we don't want to have to worry about times tomorrow - just the medal.
"[Russian rival] Yuliya Efimova will push her so she'll her need to be on her toes tomorrow, but I think there's more to come."
Briton O'Connor only reached the 200m individual medley final after Australian Emily Seebohm pulled out in order to concentrate on her preferred 100m backstroke event, but the 17-year-old found herself in second place at the halfway stage.
However, she faded in the second half to finish eighth in a race which was won by Hungary's Katinka Hosszu. Olympic champion Ye Shiwen surprisingly only managed to come fourth.
"I just need to learn how to swim it better," said O'Connor. "I keep doing that - going out too fast - and I definitely think I got a bit excited with it being a final."
Compatriot Allen, 21, added: "I'm disappointed I wasn't able to make a personal best like I did in the semi-final."
"If I'd have done that time again [2:10.23], I'd have been in fourth place but hopefully as it's my first major international final I'll be better in the future."
Scottish Commonwealth champion Robbie Renwick qualified with the fourth quickest time of 1:46.95 heading into the final of the 200m freestyle event on Tuesday and hopes to win Britain's first medal of the championships.
"I have a good lane for tomorrow night and it'll be anyone's race so I've got to get myself in a good position and really try to go for a medal," said the 25-year-old, who will face US star Ryan Lochte in the final.
Chris Walker-Hebborn, 23, and Lauren Quigley, 20, both missed out on places in their respective 100m backstroke finals, but each return to compete over 200m later in the week.
In the evening's other finals, Australia's Christian Sprenger claimed 100m breaststroke gold with South Africa's Cameron Van Der Burgh second and Felipe Lima of Brazil third.
Victory went to Sarah Sjostrom of Sweden in the 100m butterfly ahead of Australia's Alicia Coutts and US Olympic champion Dana Vollmer.
Brazilian Cesar Cielo won the 50m butterfly final.
There was frustration for Britain's Ben Proud, who did not reach the final, but would have claimed bronze with the British record of 23.10 he set at last month's trials.