However, the great Ian Thorpe predicted Hackett would soon feel the pressure.

As expected, Hackett will pin his hopes on becoming the oldest Australian swimmer to make an Olympic team in Saturday night’s 200m freestyle final after bombing out over 400m.

Hackett – 36 in May – was relegated to a distant fourth, upstaged by world-class teenager Mack Horton.

Grants Hackett competes during a heat of the Men’s 400m Freestyle. Photo: Dave Hunt, AAP.

To boot, 19-year-old Horton leapfrogged Hackett to move to second on the 400m Australian all-time list, trailing only Thorpe.

Horton clocked the world’s fastest time of the year (three minutes, 41.65 seconds) – an effort that would have won 2015 world championship gold.

Hackett (3:48.84) was not concerned, admitting his best bet for an Olympic nod was making the 4x200m relay team after his preparation was disrupted by bronchitis.

“I don’t feel any different. I just feel I know now what I need to do to get myself up and race,” he said.

“For me, it is a day at a time… I will see where that takes me – hopefully, on a plane to Rio.”

Thorpe wished Hackett the best on Thursday night, even giving his former teammate tips on how he approached his own – albeit ill-fated – London Olympic comeback bid.

However, Thorpe believed the laidback Hackett would soon feel the enormity of the task ahead.

“That’s what was the most incredible part of it [Hackett’s comeback] – how quickly it happened,” Thorpe said.

“Now for him to be able to carry it on to an Olympics, I think he will start to feel a bit more of the pressure now.

“It’s getting both the physical and mental approach right… if Grant can make this, it’s a good thing for the whole team.”

Hackett almost dropped the 400m freestyle from his program at the eight-day Olympic trials after his preparation was almost derailed by his illness.

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.

A top-two finish in the qualifying time locks in an individual Rio berth before Australian coach Jacco Verhaeren determines his relay squads.

Earlier on yesterday’s opening day, Thomas Fraser-Holmes became the first to book a ticket for Rio.

The London Olympian claimed his seventh straight national 400m individual medley title in 4:11.09.

Blair Evans (4:45.64 PB) is also Rio-bound after claiming 400m IM gold.

Four-time Paralympic gold medallist Ellie Cole (28.75) broke the world record in the 50m freestyle multi-class final.

FASTEST ALL-TIME AUSSIE MEN OVER 400M FREESTYLE:

* Ian Thorpe 3:40.08 at Manchester 2002

* Mack Horton 3:41.65 at Adelaide 2016

* Grant Hackett 3:42.51 at Fukuoka 2001

* David McKeon 3:43.71 at Adelaide 2013

-AAP