With one eye on Rio de Janeiro, swimming's stars begin the battle on Sunday for world championship medals in Kazan to prove their form heading into the 2016 Olympic Games.

The swimming section of the world aquatic championships runs from August 2-9 with hungry pretenders eager to steal some glory from reigning champions.

As ever, the USA bring a star-studded team to the Tatarstan capital, 800 kilometres east of Moscow, with marquee names including Olympic champions Missy Franklin, Katie Ledecky, Ryan Lochte and Nathan Adrian.

Treble world record-holder Ledecky is the only American to top the 2015 world rankings in her events, with the year's best times in the 400m, 800m and 1,500m freestyle.

Ledecky was responsible for breaking two of the six world records which fell in the Barcelona pool when the world championships were last held two years ago.

Franklin won six gold medals in Barcelona, but has something to prove in Kazan after failing to win an event at a Santa Clara meet in June -- her only world championships tuneup.

Likewise Lochte, who has five Olympic titles and 15 long course world championship victories, is eager to bounce back from a 2014 he called "probably my worst year of swimming."

The 30-year-old will be swimming at his sixth world championships and is looking for a fourth straight win in the 200m individual medley, but is taking nothing for granted.

"There's a bunch of other people out there," Lochte said. "Just because they're not there doesn't mean it should be a shoo-in for me."

- Sibling rivalry -

The Australian challenge will be led by two defending world champions in freestyler Cate Campbell and breaststroker Christian Sprenger.

"The difference between winning and losing can be as little as one-one-hundredth of a second," said Australia head coach Jacco Verhaeren.

"The World Championships, like the Olympics is always difficult; almost everyone is there; anything can happen and it is becoming tighter and tighter."

Campbell will get the chance to defend her world title in the women’s 100m freestyle and, if all goes to plan, she will line up for the final alongside her sister Bronte and both siblings are potential medallists.

But Netherlands' Femke Heemskerk along with Cate Campbell has posted the joint fastest time for 2015 and the Dutch swimmer will be pushing the Australian sisters all the way.

Of the teenage talent to watch out for, 18-year-old Australian Mack Horton has posted the world's fastest 400m and 800m freestyle times.

Teenage breaststroke sensation Ruta Meilutyte, who won the Olympic 100m breaststroke as a 15-year-old, is now 18 and the Plymouth-based teenager will be looking to defend her world title she won in the event from two years ago.

The Lithuanian teen set both the 50m and 100m breaststroke world records at the Barcelona championships two years ago.

At the other end of the age spectrum, 35-year-old Grant Hackett returns after an almost seven year hiatus and will swim in the 4x200m relay.

Sun Yang, the bad boy of Chinese swimming, leads his nations bid to repeat the five golds they won in Barcelona two years ago.

The Chinese giant will be looking to add the world title to his Olympic gold  in the men's 400m freestyle after a helter-skelter couple of years which also saw him banned for three months for a doping offence.

Stunned by Japan's Kosuke Hagino in the 200m at last year's Asian Games, the Chinese giant will want to prove his mettle as world champion in the 400m, 800m and 1500m freestyle.

After flopping in Barcelona in 2013 when she failed to take a medal, Ye Shiwen, the Olympic 200m and 400m individual medley champion, has something to prove on the world stage.

Hungary's Katinka Hosszu, the world champion over both distances, comes to Kazan on the back of a stellar season and will be out to attack Ye's eye-popping world 400m IM record of four minutes, 28.43secs set at the 2012 Olympics.  

- missing medallists -

Injury, illness, suspension and tragedy will rob Kazan of potential medallists.

Michael Phelps, the sport's greatest star, withdrew from USA's world championships' team after a drink-driving arrest last year.

Australia's 100m freestyle world champion James Magnussen will not be able to defend the title after shoulder surgery.

France's 200m freestyle world and Olympic champion Yannick Agnel will sit out Kazan with pleurisy.

South Korea's Olympic medallist Park Tae-hwan is serving a doping ban while Japan's Olympic bronze medallist swimmer Kosuke Hagino is also missing after fracturing his elbow in a bicycle accident.
Source: AFP