Dope-row sprinter Cesar Cielo clocked the quickest 50m butterfly as the world championships\' swimming action got under way Sunday -- and then pleaded with his critics to leave him alone. The Brazilian, who touched in 23.26sec to top qualifying, insisted he was just \"here to swim\" as storm clouds grew over his all-clear for Shanghai despite failing a drugs test. \"I don\'t know what else they want from me,\" Cielo said, after superstar Michael Phelps and Jason Lezak voiced disquiet at this week\'s controversial decision. \"I\'m a swimmer, I\'m here to swim. There\'s nothing else I can do.\" Lezak said many swimmers were unhappy after the Court of Arbitration for Sport let Cielo off with a warning for testing positive for a banned diuretic. The Brazilian has blamed a contaminated caffeine supplement for the result. The Olympic and double world champion freestyler admitted it felt \"a little weird\" to be back in the pool, but he insisted he did not \"have to make anybody else happy\". \"I\'ve had many, many emotions in the last months so it\'s kind of hard to just get everything together and just focus on swimming,\" he said. \"But I have a new page in my life and I\'m going to try and do my best and get myself into position for the final.\" China\'s Sun Yang topped qualifying in the men\'s 400m freestyle with arch-rival Park Tae-Hwan seventh, setting up an evening final which promises to be a highlight of the first day of swimming competition. Italian defending champion Federica Pellegrini led women\'s 400m freestyle qualifying, while four-time Olympic champion Kosuke Kitajima and reigning title-holder Brenton Rickard were both safely through in the men\'s 100m breaststroke. France\'s ace sprinters were fastest in the men\'s 4x100m heats, with Jessica Hardy anchoring the United States to the best time in the women\'s event.