SINGAPORE (Reuters) - The absence of a handful of the world's best swimmers, including Michael Phelps, has added an air of uncertainty to the 16th world aquatic championships, starting in Russia on Friday.
Taking place in Kazan from July 24-August 9, the championships were meant to be a full dress rehearsal for next year's Olympics in Rio but a spate of high-profile withdrawals has diluted the competition.
The highest-profile no-show is Phelps, who came out of retirement in 2014 but was subsequently banned from representing the United States at the world championships after being convicted of drink driving late last year.
Australia's James Magnussen, who won the 100 meters freestyle gold at the last two world championships, will also be missing after electing to undergo shoulder surgery, giving up his chance to become the first man to win the blue-riband sprint three times in row.
France's Yannick Agnel, the reigning world and Olympic champion for 200m freestyle, is also skipping the world championships, because of a lung infection.
Japan's Kosuke Hagino, who has emerged as one of the major threats to Phelps in the individual medley events, withdrew earlier this month after slipping over on his way to training and fracturing his elbow.
The spotlight for the men's events in Kazan is now likely to fall on South Africa's Chad le Clos and China's Sun Yang.
One of the few men to have beaten Phelps at the Olympics, Le Clos will defend the 100m and 200m butterfly titles he won at the 2013 world championships in Barcelona and is also targeting the 50m, hoping to complete a unique treble.
"This is the most important event outside of the Olympics, everything after this is just warm-ups," the 23-year-old told Reuters.
"The Olympics are like the Wimbledon tennis, you play against the same field as in other events but it is just that the prestige is greater. So the World Championships will be a vital test for me."
Sun, the undisputed king of men's long distance swimming, has been in the spotlight for all the wrong reasons since he became the first Chinese man to win an Olympic swimming gold medal at London in 2012.
In 2013, he was suspended by Chinese swimming officials from engaging in commercial activities after missing training and breaching team rules.
Later that year he was ordered to spend a week in detention after crashing a car that he had driven without a license.
In 2014, he secretly served a three-month ban after testing positive for a banned stimulant.
He could have faced a much longer penalty but Chinese doping officials ruled that Sun had made an innocent mistake and never intended to cheat.
Unfazed by the extra attention, Sun is targeting the 200m, 400m, 800m and 1,500m individual freestyle events, plus relays, a feat no other swimmer has achieved.
"When I was little, I watched Michael Phelps compete in five or six events at a worlds, so I want to try it as well," he said.
American Katie Ledecky is also trying to win the same four freestyle events in Kazan.
The 18-year-old has been smashing world records at will for the last year and seems a sure bet to win the three longer races, but the 200m is her toughest challenge.
Among Ledecky's big rivals will be her team mate Missy Franklin, who won four gold medals at the London Olympics as a 17-year-old.
Franklin won six gold medals at the last world championships in 2013 but has her heart set on seven this time, and at Rio as well.
The powerful U.S. are again expected to win the lion's share of the medals with Australia and China looming as their biggest rivals.
The opening ceremony takes place in Kazan on Friday and will be followed by a week of diving, water polo and synchronized swimming before the eight-day swimming program starts on Aug. 2.
Reporting by Julian Linden