American Rebecca Soni broke the world record in the women's 200 metres breaststroke semi-finals at the Aquatics Centre in London.
Soni, who was upset in the final of the 100m breaststroke, swam a time of 2:20.00 to dip 0.12 under the previous world mark held by Canadian Annamay Pierse.
She reclaimed the record she lost to Pierse at the 2009 world championships in Rome when the now-banned polyurethane bodysuits were still allowed.
"I felt there was no pressure. I just wanted to feel it and see how it went. I just went for it," Soni said.
"Anything can happen tomorrow so I'll save my best for that. I'm really excited, I just have to do it one more time."
Australia's Sally Foster qualified eighth fastest for the final, having finished fourth in her semi-final in 2:24.46.
Countrywoman Tessa Wallace was eighth in the same semi-final via a time of 2:27.38.