Bidding to become the oldest Australian swimmer to secure an Olympics berth, Grant Hackett surprised himself in qualify fourth fastest for the 400m freestyle final at the national trials in Adelaide.
Hackett - 36 next month - almost dropped the 400m freestyle from his program at the Olympic trials after his preparation was hampered by bronchitis.
But he cruised through the heats with plenty in reserve for Thursday night's final, clocking three minutes, 47.75 seconds.
Defending champion Mack Horton was top qualifier (3:44.90) followed Commonwealth Games silver medallist David McKeon (3:45.62).
A top two finish within the qualifying time will lock in an Olympic spot.
Dual Olympic champion Hackett said his competitive 400m effort bode well for his main event, Saturday night's 200m freestyle final.
Hackett believes his best shot at an Olympic nod is a 4x200m relay berth.
"That's a great swim. I didn't really want to go that quick - it is going to be a fast final," Hackett laughed.
"I'm an underdog for this race, but it pans out well for the 200m."
Hackett is vying for his fourth Games after coming out of a six year retirement at last year's trials and making the 2015 world titles team.
Meanwhile, Thomas Fraser-Holmes said he hoped to break his own 400m individual medley national record in Thursday night's final.
Fraser-Holmes (4:11.08) was the fastest qualifier and is vying for his seventh straight national 400m IM title.
His national and Commonwealth record stands at 4:10.14.
"If I can challenge my Australian record I will be pretty happy," he said.
"I am as fit as I have ever been, as mentally strong as I have ever been."
Fraser-Holmes is battling back to top form after the former world No.1 failed to qualify for the 2015 world titles in his pet event the 200m freestyle.
Five-time Olympic medallist Alicia Coutts is preparing for her last hurrah at the trials.
Coutts, 28, contemplated retirement after she failed to qualify for the 2015 world titles team while battling a shoulder complaint.
But she is back and looking good after qualifying second fastest (57.63) behind Emma McKeon (57.43) for Thursday night's 100m butterfly semi-finals.
She is also contesting the 100m freestyle and 200m IM.
"In two weeks' time it could be the end worse cast scenario but I am taking it as it comes," Coutts said.
Cameron McEvoy (50m butterfly), Keryn McMaster (400m IM) and Minna Atherton (50m backstroke) qualified fastest for Thursday night's other finals.