Rome, July 29 (DPA) Mary Descenza Wednesday broke the world record in the heats of the women's 200m butterfly at the world championships, while Aleksandra Gerasimenya set a new championship record in the women's 50m backstroke.

Under the blistering sun in Rome, that took temperatures into the high thirties, the 24-year-old American Descenza touched in a time of two minutes 4.14 seconds, to beat the old best mark set by Liu Zige of China last year at the Olympics. Liu's time was 2:04.18.

Descenza, who last year failed to make the American Olympic team, topped the rankings going into Wednesday evening's semi-finals from Australian Jessicah Schipper, who swam 2:05.50.

Hungary's Katinka Hosszu, winner Monday of the bronze medal in the women's 200m individual medley, was third in 2:06.21.

Liu qualified for the semi-finals with the 12th-fastest time in 2:07.55.

Gerasimenya broke the championship record in the women's 50m backstroke in a time of 27.65. Germany's world record holder Daniela Samulski was five hundredths slower, as was Russian Anastasia Zueva, who Tuesday won silver in the 100m backstroke.