Dual world champion James Magnussen has dismissed teenager Chalmers as a threat to his crack at Olympic redemption at this week’s Rio Games trials in Adelaide.

The Missile fired the first shot in what looms as the battle of the eight-day trials starting today, tipping the 100m freestyle final to be a “dog fight”.

Magnussen appears vulnerable coming into the trials in Adelaide after returning from shoulder surgery that sidelined him for nine months.

Only two individual 100m Olympic berths will be up for grabs and new rules have ensured a top-six finish in the final does not guarantee a Rio relay berth.

If world No.1 Cameron McEvoy’s stunning form holds, the other 100m spot to be decided in Monday night’s final looms as a showdown between Magnussen and 17-year-old South Australian Chalmers.

Magnussen was two seconds slower than his best in his last hit-out at last month’s NSW titles.

In contrast Chalmers broke a national record and set PBs at last week’s Australian Age Championships.

But Magnussen was at his confident best when asked about Chalmers on trials eve, claiming he was only worried about McEvoy as he plotted his Olympic revenge plan.

“Cameron McEvoy is my main competition,” said Magnussen, who was pipped by 0.01s for the London Olympic gold medal by American Nathan Adrian when a hot favourite.

“I am not really worrying about who else is in that race.”

Magnussen is no guarantee of making the team if he does not nab a top-two 100m finish within the Olympic qualifying time.

A new FINA rule prohibits a nation from bringing a relay alternate to the Olympics and not using the athlete in either the heats or final, ensuring coaches are re-thinking their strategy for Rio and considering smaller squads.

“It’s not an automatic top six [guarantees relay nod],” Australian coach Jacco Verhaeren said.

“We have to look at the quality of times first.”

Magnussen admitted he was an underdog.

“But I am quite enjoying that. I always back my ability and this year is no different,” he said.

“And I enjoy a dog fight. I thrive on that.

“If it [100m] comes down to a dog fight in the last 15 metres I will back myself every time.”

Verhaeren was not sure what to expect from Magnussen as he prepared to launch the Australian team’s campaign to crack the world No.1 ranking at Rio.

“It’s not easy to predict but we know what kind of swimmer he is,” he said.

Verhaeren has given the field a clean bill of health despite dual world champions Emily Seebohm (illness) and Bronte Campbell (hip-shoulder) hitting hurdles.

Australia fell one gold short of becoming world No.1 for the first time since 2001 when the United States pipped them at the 2015 world titles in Russia.

Verhaeren looked forward to taking the next step in Rio.

“I can’t wait to see what they have to offer [at trials],” he said.

-AAP