American teenager Missy Franklin can match the record for gold medals at a single world championships if she wins her fifth when she competes in the 100 metres freestyle in Barcelona on Friday.
The 18-year-old, who already has two relay titles and golds in 200 freestyle and 100 backstroke at the hilltop Palau Sant Jordi, is seeking to replicate a feat first achieved by compatriot Tracy Caulkins in 1978 and later by Lisbeth Trickett of Australia in 2007.
Even if Franklin fails in what she is calling her "hardest final", she will still have another two events to equal or break the record before racing ends on Sunday.
Franklin was fifth-fastest in Thursday's 100 freestyle semi-finals with a time of 53.78 seconds, slower than the personal best of 53.36 she swam in the morning qualifying session.
She has a number of in-form rivals standing in her way, including Olympic champion Ranomi Kromowidjojo of the Netherlands, Australia's Cate Campbell and Swede Sarah Sjostrom, the only swimmer to go below 53 seconds in the semis.
"If I can go a 52 that's my goal," Franklin told reporters after anchoring the U.S. to 4x200 freestyle gold.
"I want a 52 more than anything and if it puts me on the podium then that's awesome and if it doesn't that's okay. I think it's going to be a super fast final."
There are four more finals on Friday.
Rikke Pedersen of Denmark will attempt to add gold to the 200 breaststroke world record she set in qualifying on Thursday, while Tyler Clary was quickest in the 200 backstroke semis ahead of decorated American compatriot Ryan Lochte.
Daniel Gyurta of Hungary is aiming to become the first man to win three consecutive 200 metres breaststroke titles before the Americans, French and Australians start as favourites in the men's 4x200 relay.