In just three years, Chad Le Clos has transformed from fresh-faced upstart to grizzled champion with a king-sized target on his back. Le Clos became an instant star when he hunted down Michael Phelps at the 2012 London Olympics to claim the 200m butterfly crown the American legend had held since Athens, Greece, 2004.Now he is the prey. At 23, Le Clos is one of the older high-flyers on the circuit and many of his archrivals range between 18 and 21 years.story_article_left1At the 2013 world championships, Le Clos didn't merely win the 100m and 200m fly double, he toyed with the fields, looking to his left and right as he floated to victory.But he can expect a tougher time trying to keep those two titles at this year's edition in Kazan, Russia.The world aquatics championships started with diving on Friday, and the pool racing commences next Sunday.Cameron van der Burgh, the defending 50m breaststroke world champion and 100m runner-up, also faces a torrid time. The 27-year-old is ranked No2 in the 50m, but in the longer race he is joint sixth with another old-timer, Felipe Lima of Brazil, 30.Here are some swimmers Le Clos and Van der Burgh will have to watch out for:Daiya Seto (Japan), 21:For the better part of 18 months, the Pan Pacific and Asian Games champion has owned the faster times in the 200m fly. He and Le Clos met a couple of times in Europe recently, each winning one race.full_story_image_vleft1Tamas Kenderesi (Hungary), 18: At last year's European Junior Championships, he won the 200m butterfly in 1min 56.74sec to move into the world's top 20. Now he's joint No2 after going 1:54.79.Evgeny Koptelov (Russia), 21:Scooped the 100m and 200m butterfly double at the World Student Games earlier this month. His times were impressive - 1:54.79 lifted him to a share of No2 in the 200m butterfly, and 51.50 pushed him to No3 in the 100m.Konrad Czerniak (Poland), 26: This old warhorse is the top-ranked swimmer in the 100m fly at 51.37. Czerniak took silver at the 2011 world championships in Shanghai, China, and two years later he claimed bronze behind Le Clos.full_story_image_hleft2Adam Peaty (England), 20:First he relieved Van der Burgh of his 100m breaststroke Commonwealth Games crown in Glasgow, Scotland, last year, and then he broke his 50m and 100m world records. Peaty is the favourite in both events.Felipe Franca Silva (Brazil), 28: Has been winning world championship medals since Rome 2009, when he claimed the 50m breaststroke silver behind Van der Burgh. At last year's world short-course (25m pool) championships, he stunned both Peaty and Van der Burgh in both the 50m and 100m races...