The former Dunedin swimmer relocated to Auckland earlier this year.
Her 400 title defence was ruined by a false start in Sunday's heats but the 21-year-old had no trouble in the shorter distance today, leaving Olympic champion Mollie O'Callaghan in her wake at the World Aquatics Championships Arena.
Fairweathe was runner-up in the 200 at last year's championships in Doha. She charged into the Singapore semis with a time of 1:56.54s, 0.20s better than American Erin Gemmell and a half-second quicker than third-fastest O'Callaghan.
Tunisia's Ahmed Jaouadi, the 1500 freestyle short course world champion, took top seed in the men's 800 in 7:41.58, ahead of Australian Sam Short, who took silver in the 400 on Sunday.
Ireland's Olympic gold medallist Daniel Wiffen sneaked into the final along with Paris 400 champion Lukas Maertens, who beat Short for the world 400.
Italy's Simone Cerasuolo was fastest into the men's 50 breaststroke semi-finals (26.42), while American Luca Urlando was quickest into the men's 200m butterfly semi-finals (1:52.71).
Five gold medals are up for grabs in the evening session, with American great and world record holder Katie Ledecky favourite to claim a sixth 1500m freestyle crown and record-extending 22nd world title.
Olympic champion David Popovici will go for gold in the men's 200 freestyle, while Australia's backstroke queen Kaylee McKeown will battle American Regan Smith for the 100 title.
American Kate Douglass, the Olympic 200 breaststroke champion, will look to add the world 100 title to her collection.
