Olympic gold medallist Sun Yang faces a hearing at the Court of Arbitration for Sport after the World Anti-doping Agency appealed against a decision to clear him of a doping offence.
World swimming governing body Fina gave Sun a warning when he missed an out of competition test at his home in China.
Sun, 27, has won three Olympic golds, one at Rio 2016 and two at London 2012.
He was previously banned for three months in 2014 for taking prohibited stimulant trimetazidine.
Freestyle specialist Sun missed a doping test in September 2018. In January, Fina's doping panel cleared Sun of wrongdoing on the grounds that testers had breached several rules, including failing to produce authorisation letters and a nursing licence, according to Sun's lawyers. He was issued with a warning.
Sun was called a "drug cheat" by Australian swimmer Mack Horton at the Rio Olympics.