Shanghai - South African Gerhard Zandberg led the field into the semi-finals of the men's 50m backstroke event on Saturday at the 14th swimming world championships.
Zandberg, who won the bronze medal over the distance in Rome two years ago, recorded a time of 24.72 seconds.
Frenchman Camille Lacourt was second-fastest 0.31 behind Zandberg, followed by German Helge Meeuw and Swiss swimmer Flori Lang.
In the women's 50m breaststroke Jessica Hardy was the fastest in a time of 30.20. Her compatriot Rebecca Soni, who has already won two gold medals at these championships, shared the second-fastest time with Russian Yuliya Efimova.
They both swam identical times of 30.72.
Sweden's Theresa Alshammar and American Jessica Hardy were fastest in the women's 50m freestyle, both clocking 24.82.
In the absence of Germany's defending champion and world record holder Britta Steffen, who has returned to Germany after a disappointing showing in the heats of the 100m freestyle, the 50m race seems wide open.
Belarussian Aliaksandra Herasimenia, who shared the gold medal in the 100m with Jeanette Ottesen, was third-fastest, while Ottesen also qualified for Saturday's semi-finals in 13th place.
In the women's 4x100 medley relay, the US team showed that they will be the ones to be beaten in Saturday's final. Their time of 3:56.95 was more than two seconds faster than the time posted by Russia.
The Russian team came second in their heat with a time of 3:59.08, followed by China and Australia.
The finals of the women's 50m butterfly, the 200m backstroke, the 800m freestyle and the 4x100 medley relay, as well as the men's 50m freestyle and the 100m butterfly will be swum in Saturday's evening session in the Oriental Sports Center in Shanghai.