Barcelona - South Africa's Chad le Clos will battle birthday boy Ryan Lochte for men's 100m butterfly gold later on Saturday after a fourth world record fell at Barcelona's World Championships.
Russia's Yuliya Efimova, 21, shaved two hundredths of a second off the women's 50m breaststroke record, clocking 29.78secs in the morning's heats having claimed the world 200m title on Friday night.
It was the third women's breaststroke world record to fall in Barcelona and the fourth in total – all set by women – after Ruta Meilutyte in the 100m and Denmark's Rikke Moller in the 200m.
Katie Ledecky of the USA, who smashed the 1500m freestyle world record on Tuesday, is also bidding for her third gold of these world championships in Saturday's 800m final where she will battle Denmark's Lotte Friis and New Zealand's Lauren Boyle.
Efimova will be looking to add the sprint gold to her 200m world crown with the 50m semifinals on Saturday night, but both Jessica Hardy, whose record the Russian beat, and 100m world champion Meilutyte were within three hundredths of a second in the heats.
Missy Franklin of the USA is the hot favourite to win her fifth gold in Saturday's 200m backstroke final as the fastest qualifier, world-record holder, plus Olympic and defending world champion.
Having turned 29 on Saturday, USA star Lochte was the fastest into Saturday's 100m butterfly final with le Clos looking to win his second worlds gold following victory in the 200m butterfly on Wednesday.
Lochte won two golds on Friday, leaving him with three at these championships and with 15 world titles in his career, and he is second only to swim legend Michael Phelps' record haul of 26 world championships golds.
"Lochte swam really well, it is going to be a great race, I'm really excited for it," said le Clos, the Olympic 200m butterfly champion.
France's Olympic champion Florent Manaudou is the favourite in the men's 50m freestyle final, after the 21-year-old blasted out the fastest semifinal time.
"Even if the rest wore flippers, he will still be hard to beat," said compatriot Frederick Bousquet.
But Americans Anthony Ervin and Nathan Adrian, the Olympic 100m champion, plus world-record holder Cesar Cielo of Brazil, were all within a quarter of a second of Manaudou's semifinals time.
China's long-distance specialist Sun Yang is set to claim his third gold at these championships as the fastest qualifier into Sunday's 1500m freestyle final having already won the 400m and 800m world titles here.
"After the relay yesterday, I felt a bit tired, but it is okay because I have another day to relax before the final," said the 21-year-old Sun, who moved China from sixth to third place and a bronze in Friday's 4x200m freestyle relay.
Fresh from winning gold in Friday evening's 100m freestyle final, Australia's Cate Campbell was the fastest into Saturday's semifinals for the 50m sprint in the morning's heats.
"I have to be happy with that having not gotten much sleep or as much rest as I would have liked," admitted the 21-year-old, whose 100m freestyle victory added to compatriot James Magnussen's gold in the men's event.
"It's great to have the 50m after the 100m and not the other way round.
"I can stand on the top of the blocks and actually see where I am finishing.
"The 50m is great fun, it doesn't hurt nearly as much and it's splash and dash essentially."