Barcelona - Missy Franklin faces a tough fight for a fifth worlds gold in Friday's 100m freestyle final with Australia's Cate Campbell and Swedish teenager Sarah Sjostrom leading the charge.
Campbell dubbed US teen star Franklin swimming's "Wonder Woman" on Thursday after the 18-year-old picked up her fourth of a possible seven gold medals at Barcelona's world championships in the 4x200m freestyle relay.
Franklin, fifth fastest into the final, battles a strong field in the Palau Sant Jordi pool with Sjostrom, who won gold on Monday in the 100m butterfly, fastest into the freestyle final in 52.87secs with Campbell just behind her.
Olympic champion Ranomi Kromowidjojo and Germany's Britta Steffen, the world-record holder, are also vying for medals.
"I'm not expecting anything, it is a really tough field.
"I'd just love to get another best time," said Franklin.
The teenager has a punishing evening schedule with the 200m backstroke semi-finals, for which she qualified fastest, coming a quarter of an hour after the freestyle final.
"It will be my toughest turnaround, but at least the 200m back is one of my favourite events," said Franklin, with the USA men the favourites for the 4x200m freestyle final after qualifying fastest.
Having broken the women's 200m breaststroke world record in Thursday's semi-finals, Denmark's Rikke Moller Pedersen goes into Friday's final with Russia's Olympic silver medallist Yuliya Efimova as her main rival.
US star Ryan Lochte, who won his 13th world championships gold medal on Thursday in the 200m individual medley, will be battling compatriot Tyler Clary in Friday's 200m backstroke final, before doubling up in the 100m butterfly semis.
Teenage long-distance expert Katie Ledecky, who shaved nearly six seconds off the women's 1500m freestyle record on Tuesday, was the fastest into the 800m final, where the world record is under threat, from Thursday's heats.
"It was a little bit tough to come down the high of last night," she admitted after winning her second gold on Thursday in the freestyle relay.
"I was really excited and it took a little bit of time to get to sleep, but it's just a matter of recovering well."
South Africa's Chad le Clos and Russia's Evgeny Korotyshkin, who shared Olympic silver behind Michael Phelps in the 100m butterfly final last year, were the fastest into Friday's semi-finals.
Le Clos, who won the 200m butterfly world title on Wednesday, was just 0.33secs behind Korotyshkin's time of 51.55.
"It's great to already have the gold that I needed to take the pressure off," said Le Clos.
"I still want to win it, but if I don't, it's not the end of the world."
Lochte was 13th of the 16 semi-final qualifiers as he made his international debut in the event.
"This is the first time I have done this event at an international meet, so I am still learning, but anytime I get on the blocks, I feel like I can get a medal," said the 28-year-old.
Olympic champion Florent Manaudou was the fastest into Friday's evenings semi-finals of the men's 50m freestyle as he blasted out in 21.72secs.
Brazil's world-record holder Cesar Cielo was just four hundredths of a second behind, while newly-crowned world 100m champion James Magnussen was 11th fastest.
"I am going to have to be really on my game tonight to even get to the final, but if I can get there, anything can happen," said Magnussen.
USA pair Nathan Adrian, the 100m freestyle Olympic champion, and Anthony Ervin went through fifth and sixth fastest respectively, with just over half a second separating the 16 semi-finalists.
Holland's defending world champion Inge Dekker was sixth fastest into Thursday's semi-final for the women's 50m butterfly with Britain's Fran Halsall clocking the quickest time of 25.69secs, just ahead of Denmark's Jeanette Ottesen Gray, the event's world leader.
Australia's Alicia Coutts, who won silver in the 100m butterfly at the Olympics, failed to qualify as 18th fastest.
"I'd have liked to have made the semis, but it's only the 50m, it would have been worse in the longer events," said the 25-year-old, who has won four silver medals at the championships, which finish Sunday.