Sun's hearing is due to start tomorrow and the CAS expects it to last three days, after its previous ruling was quashed when a panel member's racist tweets were brought to light.

WADA is appealing International Swimming Federation's decision not to ban Sun for an alleged anti-doping rule violation in 2018, which would be the athlete's second.

Sun was initially given an eight-year ban by the CAS in February 2020, but the Chinese swimmer successfully appealed the decision to the Swiss Federal Tribunal.

Switzerland's highest court ruled that the case should be re-heard and the President of the CAS panel, Italian Franco Frattini, removed over a series of discriminatory tweets.

Hans Nater of Switzerland will act as President on the CAS panel this time, and will be joined by Jan Paulsson of France and Bernard Hanotiau of Belgium.

Due to the COVID-19 pandemic, the hearing is set to be held online via videolink, and will be conducted in private.

The previous hearing was held in public.

The case relates to a member of Sun's entourage smashing a blood vial with a hammer during an attempted doping test in September 2018.

Sun has denied any wrongdoing, claiming the officials who arrived to test him at his home on the night in question did not have the correct credentials.

Sun, an 11-time world champion and six-time Olympic medallist, previously served a three-month drugs ban in 2014 after testing positive for trimetazidine, a stimulant that was banned four months earlier.

The hearing will start with fewer than 60 days to go until the Olympics, raising the possibility of Sun competing at the Games before the CAS renders a verdict.

Last time, the CAS took more 100 days to announce a verdict.

Sun's first CAS hearing was plagued with translation issues, leading to an interpreter appointed by his team being replaced by a WADA staff member.

His behaviour during the hearing - the first held in public by the CAS this millennium - was also criticised by some observers.