Brazilian swimming star Cesar Cielo is fighting to avoid a doping ban which could wreck his Olympic chances in a case which has rocked the sport and overshadowed the world championships.
The 24-year-old Olympic and double world champion went into a Court of Arbitration for Sport hearing in Shanghai along with three team-mates who also tested positive for a banned diuretic in May but were let off with just a warning.
Cielo has blamed a contaminated food supplement for testing positive for furosemide, which is sometimes used to mask other drugs and is banned by the World Anti-Doping Agency.
Governing body FINA has tightened its anti-doping procedures at the Shanghai championships by reintroducing blood testing, alongside urine sampling, for the first time since 2005.
Swimmers 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.