Chinese swimmer Sun Yang has rekindled hopes of returning to the pool at the delayed Tokyo Olympics after having his eight-year ban for a controversial doping violation from the Court of Arbitration for Sport overturned by a Swiss court.

The Swiss Federal Tribunal upheld Sun's appeal against the career-ending ban issued in February by the Switzerland-based CAS, which ruled that Sun had violated doping control regulations by refusing to cooperate during a random test in September 2018 at his home in Hangzhou, Zhejiang province, and has ordered the case to return to the CAS for a potential second trial, Xinhua News Agency reported on Thursday citing anonymous sources.

The tribunal hasn't commented publicly on the decision yet, but the World Anti-Doping Agency, the party opposing Sun in the case, confirmed in a statement on Wednesday that the Swiss Federal Tribunal had upheld a "revision application" filed by Sun's legal representatives and had "set aside" his eight-year suspension from swimming.

Although not confirmed by any parties involved, the reason behind the reversal was reportedly a successful challenge by Sun's lawyers that questioned the neutrality of Franco Frattini, chairman of the three-person CAS panel that issued the ban.

Sun's lawyers specifically pointed to past social media posts from Frattini that carried a strong sentiment against alleged animal cruelty in China, said a Yahoo report.

Sun's case regarding the 2018 drug test was first heard by the swimming's world governing body FINA, which cleared him of any wrongdoing in January 2019 after an internal hearing.

The World Anti-Doping Agency disagreed and appealed the decision in March that year to the CAS, culminating in a livestreamed hearing in November that was marred by frequent translation issues.

The CAS ultimately ruled that Sun was guilty and issued the ban.

Sun, a three-time Olympic champion swimmer, reiterated his innocence during the CAS hearing, claiming that he refused to cooperate during the test only after finding sample collectors from IDTM, a FINA-hired agency, operating without adequate identification and authorization.

Having won two gold medals in the 400 and 1,500 meters freestyle at the 2012 London Olympics and the 200 meters freestyle at the 2016 Rio Olympics, Sun was considered China's biggest gold medal hope at the delayed Tokyo Olympics, which will open on July 23 next year.

The potential second CAS hearing will decide whether Sun will be eligible in time to catch up with the delayed qualification program for the Tokyo Olympics.

"Sun Yang's case will return to the CAS under a panel with a new chairman," WADA Vice-President Yang Yang said in a China Central Television interview on Thursday.

"The WADA has now only received a short message instead of more details. There will be further communication once the WADA has more information," said Yang, who is also a retired Winter Olympic champion skater.

However, Sun's journey back to the sport won't be easy, as the WADA has made it clear that it will present its case "robustly" again.

"In the CAS award, the WADA clearly prevailed on the substance of the case as it was able to show that there were a number of aspects of the original FINA decision that were incorrect under the World Anti-Doping Code and the related International Standard for Testing and Investigations," said the WADA statement.

"The WADA will take steps to present its case robustly again when the matter returns to the CAS panel, which will be chaired by a different president."

The news became a trending topic on Chinese social media, with the hashtag "Swiss court overturned Sun Yang's case" viewed over 230 million times by Thursday afternoon on Weibo. Most fans expressed their support for Sun to continue his legal fight and hoped to witness the star shine again at the Tokyo Olympics.

"I firmly believe in my innocence. I was shocked and angry, and I'm unable to understand the CAS's decision," Sun told media earlier this year. "I will definitely appeal to let more people know the truth. I believe the facts will eventually defeat lies."