The Swiss Federal Tribunal has upheld Sun Yang's appeal against an eight-year doping ban given to the Chinese swimmer by the Court of Arbitration for Sport (CAS) earlier this year.
Sun's upheld appeal related to CAS panel President Franco Frattini - a former Italian Foreign Minister.
The World Anti-Doping Agency (WADA) said that it had been informed of the decision to overturn Sun's ban, but had not yet received a full reasoned decision.
A series of tweets from Frattini about China's dog-meat trade came to light several weeks after the CAS ruling, including one where Frattini uses a racial slur.
The case is now poised to be heard by the CAS once more, with a different panel in place.
In February, Sun was banned for eight years because a member of his entourage smashed a blood vial with a hammer during an attempted drugs test in September 2018.
The International Swimming Federation (FINA) had initially let Sun off with a warning, but WADA appealed that sanction to the CAS.
Following a 10-hour public hearing in November of last year, the CAS handed down a verdict of an eight-year ban - the maximum possible sentence.
It appeared to rule Sun out of the Tokyo 2020 Olympics and effectively end the 29-year-old's career, only for Switzerland's highest court to now set the ruling aside.
"The Swiss Federal Tribunal’s decision upholds a challenge against the Chair of the CAS Panel and makes no comment on the substance of this case", WADA said in a statement.
The Swiss Federal Tribunal only rules based on potential procedural issues and on human rights, and this is not a judgement on the CAS panel's interpretation of the law.
Sun, an 11-time world champion and three-time Olympic gold medallist, has always denied wrongdoing and claimed the officials who arrived to test him at his home on the night in question did not have the correct credentials.
Given FINA did not initially suspend Sun for the offence, it would appear that he is now free to compete once more.
WADA said it "will take steps to present its case robustly again when the matter returns to the CAS".
The first CAS hearing was punctuated by issues with translation, resulting in an interpreter appointed by Sun's team being replaced by a WADA staff member, and Sun's behaviour was criticised by some observers.
The swimmer even tried to invite a member of the gallery into proceedings to translate his closing statement, irking panel President Frattini.
It was the first public CAS hearing this millennium.
The last public CAS hearing, curiously, also concerned aquatics, and was between Irish swimmer Michelle Smith de Bruin and FINA in 1999.
A gold medallist in the 400-metre and 1500m freestyle races at London 2012, and the 200m freestyle champion at Rio 2016, Sun has proven a controversial character in the swimming world and served a three-month drugs ban in 2014.
At the 2019 FINA World Championships, Australian Mack Horton and Britain's Duncan Scott both refused to share the podium with Sun, and there was an angry exchange between Scott and the Chinese swimmer.