China's Sun Yang has lodged an appeal to the Swiss Federal Tribunal in an attempt to overturn his eight-year doping ban.
In February, the Court of Arbitration for Sport (CAS) found the three-time Olympic champion guilty of refusing to cooperate with sample collectors during a visit to his home in September 2018 that turned confrontational.
A spokesperson from Switzerland's highest court told ESPN that Yang's appeal had been registered on April 29.
The original case went to trial World Anti-Doping Agency (WADA) appealed against a ruling by swimming governing body FINA to simply warn Sun over the confrontational incident.
The ban means that the swimmer cannot defend his 200 metres freestyle title at the Tokyo Olympics which have been postponed until next year due to the coronavirus pandemic.
Sun also served a three-month ban in 2014 imposed by Chinese authorities after testing positive for a stimulant that was banned at the time, but the sanction was not announced until after it ended.
The 28-year-old has become a controversial figure among fellow swimmers and two competitors refused to stand with him on medal podiums at the 2019 world championships.
Sun is one of the biggest stars in the sport. He added another gold medal at the 2016 Rio de Janeiro Olympics to the historic 400m and 1,500m titles he won at the London Games.
He won 11 golds in five consecutive world championships from 2011 to 2019, at each freestyle distance from 200m to 1,500m. At three Asian Games from 2010 to 2018, he won nine gold medals.