Brisbane: Australian middle-distance queen Ariarne Titmus has fired a warning shot to American superstar Katie Ledecky with a new Commonwealth record in the 400-metre freestyle at world championship trials on Sunday.

The 18-year-old, seen as Ledecky’s biggest challenger at the worlds in South Korea next month, dipped under the magical four-minute mark to clock 3:59.35 and secure her place on the team.

Only two spots for the championships are available in each individual event being contested at the Brisbane Aquatic Centre over six days, with Kiah Melverton (4:05.30) joining her. Asked if she could go quicker in Gwangju, Titmus replied: “Definitely”.

“The last 100m was a bit of a sting, but I felt really good in the first 200,” added Titmus, currently ranked world No. 1 in both the 200 and 400-metre freestyle.

She will also contest the 200 and 800 metres in Brisbane. In other races, Olympic champion Mack Horton, who beat China’s Sun Yang to win the 400-metre freestyle gold at Rio in 2016, was upset by Jack McLoughlin, who swam a personal best 3:44.34 to clinch their showdown.

Australia’s most successful Rio swimmer Emma McKeon — who won gold, two silver and a bronze — was the fastest in the women’s 100-metre butterfly (57.28), followed by Brianna Throssell (57.57).