KAZAN, Russia (AFP) - Olympic champion Nathan Adrian of the United States admits he is disappointed not to be ale to resume his 100m freestyle battle with injured world champion James Magnussen in Kazan.

Adrian won the Olympic title by just one-hundredths of a second when he beat Magnussen on the wall in London three years ago with "The Missile" having to settle for silver.

The Australian, who gained revenge by winning the world title in Barcelona two years ago when Adrian collected bronze, misses the 2015 world championships with a shoulder injury.

The swimming section of the world aquatic championships starts here on Sunday and there will be a new world 100m freestyle champion when the men's heats start on Wednesday with the final on Thursday night.

"It's a shame to ever see anyone go down. I'm sure it probably eats him inside to be at home right now - that sucks," said Adrian in Thursday's USA team press conference.

"It's not something I'd wish on anybody, I wish him all the best and that he recovers quickly.

"But James is a pretty resilient guy. He'll figure out how to go fast in the past and I'm sure he'll do it again."

Even without his main rival in Kazan, Adrian knows the mantle of being the event's Olympic champion will count for nothing once the heats get under way.

He can expect a fast field in Kazan as his rivals stake a claim for a world title in the last major competition before the Rio de Janeiro Olympic Games.

As the US trial was held last year, Adrian, who also goes in the 50m freestyle, has watched his rivals clock fast times at their respective national trials on the road to Kazan.

Canada's Santo Condorelli, Russia's Vladimir Morozov and Australia's Cameron McEvoy have all swum around the 48-second mark this year, close to the speed which won world medals in Barcelona two years ago.

"It's hard to narrow it down, this field is going to be really fast," said Adrian.

"There are a lot of people who have been under 48 seconds either this year or last.

"Vlad's good, Santo and Cam have been consistent in doing that too.

"There is a field of a lot of guys who can go under 48, it's just a matter of who goes furthest under to win.

"A lot of people have had their trials, gone to continental championships and done well there, but this is where it counts."