|Missy Franklin of the U.S. reacts after winning the women's 200m freestyle final during the World Swimming Championships at the Sant Jordi arena in Barcelona July 31, 2013. REUTERS/Albert Gea|
The 18-year-old has said she expects the 100 freestyle final, scheduled for 1602 GMT (5.02 p.m. British time), to be her toughest race at the hilltop Palau Sant Jordi and will have only around 20 minutes to recover before the 200 backstroke semi-finals.
Franklin has won two relay titles and golds in 200 freestyle and 100 backstroke and if she managed a fifth triumph she would match a feat achieved only by compatriot Tracy Caulkins in 1978 and Australian Libby Trickett in 2007.
No woman has won more than five golds at a world championships - East German Kristin Otto won six at the 1988 Seoul Olympics - and Franklin is also due to swim for favourites the United States in Sunday's 4x100 medley relay.
"Today is kind of my last really hard day, I have three events and then just two more races," Franklin told reporters.
"I am really excited it's kind of starting to wrap up and I am so excited about this 200 back, I've been waiting for it this whole meet," she added.
"I think it will be my toughest turnaround tonight but the good news is that the 200 back is one of my favourite events.
"So I'm just going to go out there and swim the 100 (freestyle) and see what I can do there, get a little shakedown and then come right back."
Franklin's team mate Katie Ledecky, who smashed the world record to win 1,500 freestyle gold on Tuesday after taking the 400 title on Sunday, posted the fastest time in the 800 heats ahead of New Zealand's Lauren Boyle and Dane Lotte Friis.
The 16-year-old Ledecky, part of Thursday's victorious U.S. 4x200 freestyle relay team, can claim a fourth gold in Saturday's 800 final, her last race of the week.
In Friday's other heats, South Africa's Chad le Clos was second-quickest in the 100 metres butterfly behind Russian Evgeny Korotyshkin before he seeks a second gold in Saturday's final to add to Wednesday's 200 metres butterfly title.
"There's a lot of guys who have a chance of winning the gold," Le Clos told reporters.
"It's such a hard race to call but I'm also very excited," added the 21-year-old.
Le Clos said his 200 butterfly gold had taken some of the pressure off.
"I feel more relaxed than I was for the 200 because now I've got the gold that I needed to get so this one if just like a bonus race for me," he said.
"Obviously I want to win it but if I don't it's not like the end of the world I've still got a gold."
Frenchman Florent Manaudou, the Olympic champion in London last year, was quickest in the heats for the men's 50 metres freestyle ahead of world record holder Cesar Cielo of Brazil.
Australian James Magnussen, gold medallist in the 100 freestyle on Thursday, was equal 11th-quickest.
There was something of a surprise in the heats for the men's 4x200 freestyle relay when Australia failed to qualify for Friday evening's final.
(Editing by Ed Osmond)