US swimming great Michael Phelps returned to China Wednesday looking for the form that swept him into the history books at the Beijing Olympics, as a major doping row clouded the world championships. Phelps, who has caused concern with some stuttering swims this year, made a low-key arrival at Shanghai\'s Pudong Airport, looking unshaven and wearing a baseball cap. He did not speak to media and was whisked away on a USA team bus. \"Weeeee twitter DOES work!!! Here at last,\" he tweeted afterwards, referring to China\'s ban on the popular microblogging site. Phelps could take part in up to seven events in Shanghai, which would give him a shot at eclipsing the five gold medals he won in 2009 as he tries to kick-start his preparations for next year\'s London Olympics. The Baltimore native already has 14 Olympic titles at the age of just 26, including the eight he won at Beijing\'s Watercube -- the greatest performance Olympic swimming has ever seen. And the Shanghai championships, whose diving, water polo and synchronised swimming are well under way at gleaming new venues, would welcome some Phelps heroics after a high-profile dope case hovered over the opening week. On Wednesday, Brazilian sprinter Cesar Cielo, one of the sport\'s biggest stars, attended a six-hour Court of Arbitration for Sport tribunal which will decide whether to hand out a ban that could wreck his Olympic chances. Cielo, wearing a dark suit and sunglasses, smiled but did not speak to media as he left with three team-mates who also tested positive for a banned diuretic in May. All four were let off with a warning by Brazilian officials. The 24-year-old Olympic and double world champion has blamed a contaminated food supplement for testing positive for furosemide, which is sometimes used to mask other drugs and is proscribed by the World Anti-Doping Agency. Governing body FINA has asked CAS to suspend the swimmers and annul their results since May. The court has promised to hand down a judgement by Friday in the case, before the championships\' swimming events start on Sunday. FINA has also tightened its anti-doping procedures at the Shanghai worlds by reintroducing blood testing, alongside urine sampling, for the first time since 2005. And athletes at the world championships have been urged only to eat at designated restaurants in Shanghai to avoid meat contaminated with banned clenbuterol, which Chinese farmers sometimes feed to pigs and cattle. In Wednesday\'s action, Greek swimmer Spyros Gianniotis dethroned German title-holder Thomas Lurz -- the unofficial king of open water swimming -- to win the 10km crown, his first world championships gold. Gianniotis, who has repeatedly lost out to Lurz at major events, finally topped the podium after touching in 1hr 54min 24.7sec in baking conditions at Jinshan City Beach. \"I\'ve trained several years for that. It feels good,\" he said. \"It was very hard these last two months because I had an injury. I tried to never stop swimming and I did it.\" Competitors were tracked with high-tech sonar equipment during the gruelling event to address safety concerns following the death of America\'s 2009 10km bronze medallist Fran Crippen during a race last year. In diving, China\'s Hu Yadan and Chen Ruolin led the women\'s 10m semi-final standings as the hosts pursue a sweep of all 10 gold medals, with six already in the bag. And Russian synchronised swimming queen Natalia Ishchenko celebrated the 14th title of her record-breaking world championships career with gold in the solo free event.