BARCELONA Yuliya Efimova of Russia set a world record in the 50 meters breaststroke and Australia's Cate Campbell left her rivals for 50 freestyle gold trailing in qualifying at the world championships in Barcelona on Saturday.
Efimova, who said she had barely slept after Friday's gold in the 200 breaststroke and was stunned by her performance, touched in a time of 29.78 seconds to beat the previous best of 29.80 set by American Jessica Hardy in 2009.
Hardy was swimming alongside the Russian and posted the second-quickest qualifying time of 29.99 seconds.
"I still don't understand it," Efimova told reporters.
"Yesterday I was ready for a world record but today, I didn't sleep half the night and I am so tired," the 21-year-old added.
"I didn't do the warm-up and I am so sleepy and I just swam a record it's crazy."
Campbell, whose bid for Olympic gold in the freestyle sprints was wrecked by illness in London last year, posted 24.27 seconds in the 50 meters version on Saturday, well ahead of Briton Francesca Halsall in second with 24.60.
Campbell's younger sister Bronte was third quickest in 24.65 ahead of Ranomi Kromowidjojo of the Netherlands, Olympic champion at 50 and 100 meters, in 24.68.
Campbell put some of the disappointment of missing out at the Olympics behind her by winning the 100 gold on Friday and said she was still recovering.
"I'm stinging a little bit I won't deny but did what I needed to do, got through fairly comfortably so hopefully I can just build on that," she told reporters.
"I didn't get that much sleep or probably as much rest as I would have liked so I am really happy with that swim," added the 21-year-old.
"It's a 50, how much energy can you blow? 50's are fun. They don't hurt. It's splash and dash, essentially."
Sun Yang of China, chasing a rare treble of distance freestyle golds and already the winner of titles at 400 and 800 meters in Barcelona, eased through as the fastest man in qualifying for Sunday's 1,500 final.
Sun, who set the world record at the longer distance to win Olympic gold in London, will match a feat achieved previously only by Australian Grant Hackett in 2005 if he wins all three world titles as expected.
Daniel Orzechowski of Brazil topped the heats for the 50 backstroke ahead of Spain's Aschwin Wildeboer and Frenchman Jeremy Stravius.