Adelaide - Two-time world champion James Magnussen and dual Olympic champion Grant Hackett face major challenges to qualify for their individual events at Australia's Olympic swimming selection trials starting in Adelaide on Thursday.
Magnussen, who was beaten by 100th of a second for gold in the 100m freestyle at the 2012 London Olympics, is fighting back from a shoulder injury and trails top-ranked team-mate Cameron McEvoy and rising youngster Kyle Chalmers heading into the cut-throat trials.
The heat is on Magnussen, once the fastest swimmer in the world but now facing a battle to squeeze into the two top qualifying places in his favoured event.
McEvoy is at the top of the world rankings with his 47.56 winning this year's Aquatic Super Series in Perth, while 17-year-old Chalmers broke the 50m national record and set a 200m personal best in world class times at the Australian Age Championships in Adelaide last week.
"His times are absolutely impressive," Australia's head coach Jacco Verhaeren said of Chalmers. "Does he have the talent for it (Olympic selection)? Of course. Is it possible right now? We will see this week."
Dual Olympic champion Hackett, 35, who came out of six years of troubled retirement to try to make the Rio Games, is battling lingering illness to shoot for an individual swim in the 400m freestyle.
Hackett, who retired in 2008 after winning the 1 500m freestyle at both the 2000 Sydney and 2004 Athens Olympics, and claiming four world titles in the event, may have to focus instead on a place in Australia's 200m freestyle relay team.
"I need everything to go right given I've had six years out of sport and how good these guys are now and what I'm up against," Hackett said of the 400m.
"I am in a position with nothing to prove and nothing to lose so just see how I go."
Cate and Bronte Campbell will resume their sibling rivalry in the women's freestyle sprints after Bronte cleaned up with wins in the 50m and 100m at last year's Kazan world championships.
She has been troubled by hip and shoulder problems this year and needs a cortisone injection to settle her shoulder, but has not been able to risk time out of water so close to the trials and will instead wait until immediately after racing to seek a full diagnosis.
"It has definitely not been the best preparation of my life but I will draw on past experience and I think I've got enough work done to put together a good race and that is what I need to do," she said.
"Trials are about getting on the wall first or second and making sure you're on the team. It's pretty cut-throat and there is no room for error."
Cate, the 100m freestyle world champion in 2013, battled injury and shoulder surgery in late 2014 which left her underdone last year when she claimed bronze behind Bronte in the event at Kazan.
World champion Mitch Larkin has his eyes set on the 100m backstroke world record at the Australian trials. His best time of 52.11 is just 0.17 short of US great Aaron Peirsol's 2009 record of 51.94.
Larkin is the reigning world champion in the 100m and 200m backstroke and he is looking to clinch his spot in both events at Rio.