MELBOURNE: Two months on from Summer McIntosh's world record swims, Olympic champion Ariarne Titmus will have a chance to respond to the Canadian wunderkind's challenge at world championship trials in Melbourne next week.The rivalry between Titmus and US great Katie Ledecky has ignited swimming in recent years but the pair face a new threat in 16-year-old McIntosh, who smashed Titmus's 400m freestyle world mark with a time of 3 minutes, 56.08 seconds at the Canadian swimming trials.The idea of a 400m showdown between the trio at next year's Paris Olympics is being billed as a potential "race of the century", 20 years after Ian Thorpe edged Pieter van den Hoogenband and Michael Phelps at the 2004 Athens Games.Titmus swam a modest 4:00.49 to retain her 400m title at national championships in April but has been building towards a performance at the Melbourne trials starting on Tuesday (Jun 13)."As a competitive person, anybody swimming fast in the events that you compete in are part of that competitive motivation," Australia's head coach Rohan Taylor told Reuters on Friday of the 400m rivalry."Summer's an exceptional athlete who has been doing some fantastic things and that's what makes our sport engaging."As far as Arnie (Titmus) goes, she's shown her competitive desire to be the best and I would assume (McIntosh) would motivate her to continue to do that."There is enough competition in the home pool to keep Titmus on her toes, with teenager Mollie O'Callaghan to renew their budding rivalry in the 200m freestyle.O'Callaghan pipped Titmus to the national 200m title in April, adding to her victory in the 100m sprint and 50m backstroke.The 200m backstroke world record-holder Kaylee McKeown and Emma McKeon, who scooped four