Simmonds improved on the 50m backstroke time she set in the morning
Lizzie Simmonds set her fifth British record of the European Short-Course Championships in Croatia on Saturday.
The 17-year-old came fifth in the 50m backstroke to lower the UK record she set in the morning to 26.88 seconds.
However, race winner Sanja Jovanovic of Croatia stole the show with a world record time of 26.23secs.
"It's always great to be in a race with a world record," said Simmonds. "It just shows you how swimming is moving forward which is always a great thing."
There were other world records for Russia's Nikolay Skvortsov and France's Amaury Leveaux.
Skvortsov won the 200m butterfly in 1: 50.60, with Britain's Michael Rock in fourth and Joe Roebuck seventh.
Leveaux became the first man to break the 45-second barrier in the 100m freestyle, clocking 44.94secs.
Briton Richard Charlesworth was eighth in the 1,500m freestyle, while Marco Loughran qualified seventh fastest for Saturday's 100m backstroke final after smashing his PB for the second time in a day in 51.87secs.
James Goddard, the 200m individual medley champion, qualified second fastest for the 100m individual medley final, but James Gibson failed to qualify for the final of the 50m breaststroke.
Jemma Lowe made the 100m butterfly final in sixth position in 57.44secs.