In morning prelims.
"I still don't understand it," the Russian said after breaking the mark in the 50-meter breaststroke Saturday at the world championships. "Yesterday I was ready for a world record. But today, I didn't sleep half the night and I am so tired. I didn't do the warm-up and I am so sleepy — and I just swam a record. It's crazy."
Efimova clocked 29.78 seconds in the one-lap race, shaving 0.02 off the previous record set by American Jessica Hardy in 2009 at the height of the rubberized suit era.
Efimova could hardly believe it when she saw her time, dropping her mouth wide open in amazement as she broke into a wide smile.
Hardy qualified second in 29.99, and 100 breaststroke world record holder Ruta Meilutyte of Lithuania advanced third in 30.07.
Efimova won gold in the 200 breaststroke Friday after taking silver in the 100. She treated the 50 lightly, as it's not an Olympic event.
"I always like to swim the 200 but actually I am a short sprinter, and for me it's like my birthday," she said. "It's like my holidays ... It's always fun."
There could be more fun in the semifinals later, with Efimova, Hardy and Meilutyte each capable of swimming another record.
The 21-year-old Efimova trains in Los Angeles under U.S. coach and breaststroke guru Dave Salo.
"She's really maturing as an athlete," Salo said. "Years ago, I think she just did it because she was good at it. Now I think she's taken a bit more control over it. She understands now how she good she is. Breaking the world record doesn't happen very often."
Training under Salo means Efimova can practice with Hardy and three-time Olympic champion Rebecca Soni, who is taking the year off.
"I'm not going to learn her language. She's learning my language," Salo said. "And so the more and more she's learned my style of coaching, my language, I think that relationship has gotten better and better. ... She's got some great speed as well as endurance."
Also on the penultimate day of competition, Cate Campbell put herself in position to pull off a sprint double.
A day after taking gold in the 100 freestyle, the Australian led the 50 free prelims with a quick 24.27 seconds.
"It's great standing up on the blocks and looking down the pool and be like, 'I can see where I finish,'" Campbell said. "I'm really thrilled it's the 100 and then the 50 and not the other way around — 50s are fun. They don't hurt. It's splash and dash, essentially."
Francesca Halsall of Britain qualified second in 24.60. Campbell's younger sister, Bronte, advanced third in 24.65.
Olympic champion Ranomi Kromowidjojo of the Netherlands was fourth in 24.68, and American veteran Natalie Coughlin got through to the semifinals in ninth place in 25.00
In other prelims, Chinese standout Sun Yang led the marathon-like 1,500 free in a relaxed 14:54.65, more than 20 seconds off the world record he set at last year's London Olympics.
And Daniel Orzechowski of Brazil led the non-Olympic 50 backstroke in 24.67.
Meet standout Missy Franklin had the morning off.
Franklin will later aim for her fifth gold of the competition, in the 200 backstroke final, and fellow American teenager Katie Ledecky will bid for a world record in the 800 free.
Finals are also scheduled in the men's 50 free and 100 butterfly and the women's 50 fly.