Dual Olympic champion Ariarne Titmus has laid down an ominous marker in a heat swim at the Australian championships.
Titmus logged the second-fastest 200 metres freestyle time in the world this year while barely extending herself in a Friday morning heat in Adelaide.
Titmus clocked one minute 55.53 seconds and now owns the two quickest times in the event this year.
Her Friday heat swim was 1.13 seconds slower than her time at the NSW state championships in March.
Titmus won 200m and 400m freestyle gold medals, and a silver in the 800m freestyle, at last year's Olympics in her duels with American great Katie Ledecky.
In other Friday morning heats at the nationals, Zac Stubblety-Cook followed his world record in the 200m breaststroke by being fastest through the 100m breaststroke heats.
Stubblety-Cook on Thursday night won the 200m breaststroke final in 2:05.95, eclipsing the previous benchmark of 2:06.12 of Russia's Anton Chupkov set in 2019.
The 23-year-old became the second Australian swimmer to hold a world record, joining Kaylee McKeown who is the fastest-ever in the women's 100m backstroke.
McKeown on Friday morning cruised through her 100m backstroke heat in 1:00.24, well outside her landmark time of 57.45.
Other fastest qualifiers in Friday morning heats included Brendon Smith in the men's 200m individual medley, Isaac Cooper in the men's 50m backstroke and Jenna Strauch in the women's 200m breaststroke.
The national titles double as the selection meet for next month's world titles and the Commonwealth Games in July-August.
The top two swimmers in finals earn a world championship berth with the top three placegetters qualifying for the Commonwealth Games in Birmingham.
