Barcelona - South African Chad le Clos and American teenager Missy Franklin were on form in Friday's early heats at the 15th FINA world championships.
Le Clos, who famously beat superstar Michael Phelps at the London Olympics in the 200m butterfly and won the event in Barcelona on Wednesday, won his heat in the 100m butterfly to qualify for the semi-finals with the second-fastest time.
Only Russian Evgeny Korotyshkin, who won his heat in 51.55 - 0.33 seconds faster than Le Clos - had a better time. Hungarian Laszlo Cseh had the third-fastest time.
Franklin, who has already won four gold medals in the Palau Sant Jordi, topped the qualifiers from the heats in the women's 200m backstroke.
The 18-year-old swam a 2:07.57, beating the time set by Australian Belinda Hocking by 0.07.
Australian Alicia Coutts, who won silver at the last two 100m butterfly world championships, surprisingly failed to advance to the semi-finals in the 50m butterfly.
Britain's Francesca Halsall tied with Dane Jaenette Ottesen Gray for the fastest time in the heats. Both swam a 25.69, followed by China's Lu Ying.
Defending champion Inge Dekker, who is hoping to become only the second woman after compatriot Inge de Bruijn to achieve back-to-back victories in 50m butterfly, qualified for the semi-finals with the sixth-fastest time.
Olympic champion Florent Manaudou led the field into the semi-finals of the men's 50m freestyle. The Frenchman won his heat in 21.72 - 0.04 faster than Brazilian Cesar Cielo, who is looking for his third consecutive world championship title over the distance.
Russian Andrii Govorov was a further 0.04 behind.
American Nathan Adrian, who swam the season's best (21.47), qualified with the sixth-fastest time, while 100m freestyle champion James Magnussen shared 11th spot with veteran South African sprinter Roland Schoeman.
Schoeman, 33, first medalled over the distance at the worlds in Fukuoka in 2001 and won the 50m free in 2005. He qualified for Friday night's heats in 22.04.
The finals of the men's 100m freestyle, 200m breaststroke as well as the 4x200m freestyle relay and the women's 200m backstroke and 200m breaststroke are scheduled for Friday evening's session. - Sapa-dpa