Rebecca Adlington produced a sprint finish to win the 800m freestyle at the World Championships in Shanghai.
The 22-year-old - who won silver in the 400m last week - was behind with 100m to go but overhauled Lotte Friis.
Adlington had never won a world title but held on to claim Britain's fourth medal of the meeting.
Earlier Michael Phelps won his 25th world title by winning the 100m butterfly in 50.71 seconds ahead of Konrad Czerniak and Tyler McGill.
Double Olympic champion Adlington and Friis were together from the start before the Danish swimmer made a break after 500m.
World record holder Adlington was 0.65 seconds behind with two lengths remaining but managed to sprint clear to win in a time of eight minutes 17.51 seconds.
She told BBC Sport: "Lottie is always such a strong competitor that I knew it was going to be a battle.
"I'm so glad I've added to my collection. She is just an absolute competitor and always there battling.
"Next year it will be the two of us again but hopefully I'll have the crowd behind me.
"I don't think I can do the 200m in London as I don't have enough speed. I want to work on the 400 and 800m, they are the ones I prefer.
"But hopefully I will get a spot on the 4 x 200m relay team."
Phelps was only third at the halfway stage of his final but powered back down the second 50m to take his third gold of the week.
Defending champion Liam Tancock won his semi-final of the 50m backstroke in 24.62 seconds to qualify fastest for Sunday's final.
The Brit told BBC Sport: "I want to come here and race fast in every race and now I've got my spot in the final.
"It is a very tough field and medals could come from every lane but I am in there with a shot."
Fran Halsall, who finished joint-fourth in the 100m, comfortably qualified for Sunday's 50m freestyle final after finishing second in her semi-final behind Herasimenia Ranomi Kromowidjojo.
Halsall was only able to resume training in March after undergoing ankle surgery last December.
Kate Haywood missed out on a place in the 50m breaststroke final by 0.02 seconds after finishing fifth in her semi-final, which was won by Russian Yuliya Efimova.
Lizzie Simmonds finished seventh in her 200m backstroke final which was won by 16-year-old American Melissa Franklin.
Inge Dekker won the women's 50m butterfly in a time of 25.71 seconds while Brazilian Cesar Cielo Filho defended his 50m freestyle in 21.52 seconds.
Daniel Fogg could only manage the 14th fastest time in the 1500m freestyle heats, finishing in 15 minutes, 13.39 seconds. China's Sun Yang led the way in 14:48.13.
The women's 4x100m medley relay team, consisting of Haywood, Georgia Davies, Jemma Lowe and Amy Smith, finished sixth as winners the United States narrowly missed out on a world record.