Swimming star Sun Yang has refused to give up on his career despite seeing his hopes of defending his Olympic title at the Tokyo Games shattered by a suspension of over four years issued by the Court of Arbitration for Sport.

The Lausanne-based CAS on Tuesday announced that an earlier eight-year ban on Sun for a doping-control violation has been reduced to four years and three months, citing rule amendments that allow flexibility to assess the "entirety of the circumstances".

The latest ruling ultimately means Sun cannot defend his 200m freestyle title at the delayed Tokyo Olympics, which open on July 23.He will be eligible to compete again in May 2024, in time for the Paris Olympics, when he will be 32.

The chance of a possible comeback in Paris means Sun is reluctant to call it quits, at least for now.

"I think I will pull through to the Paris Games," Sun said in a text reply to freelance reporter Yang Wang, who posted the message on Weibo on Wednesday.

"I am actually in pretty good form, and I am confident (for the future)," added Sun, who became the first Chinese man to win an Olympic swimming gold medal after claiming the 400m and 1,500m freestyle titles at London 2012.

However, with age no longer on his side, Sun would face huge challenges, both in and out of the pool, to reboot his career.

His controversial doping case dates back to September 2018 when a dispute with testers from IDTM, a FINA-hired agency, over the authorization of a random out-of-competition drugs test at his residence in Hangzhou, Zhejiang province escalated into a physical confrontation.

Swimming's world governing body FINA ruled in January 2019 that Sun's refusal to cooperate was reasonable based on results of an independent panel's investigation that found the IDTM testers failed to show adequate proof of identification and authorization, as claimed by Sun.

Two months later, the World Anti-Doping Agency appealed the FINA ruling to CAS, resulting in an 11-hour livestreamed public hearing in November 2019 in Swiss city Montreux. There, a panel of three judges determined Sun was guilty and imposed the eight-year ban in February 2020.

The Swiss supreme court set aside the ban in December of that year following a successful appeal by Sun's attorney that challenged the neutrality of Franco Frattini, chairman of the original CAS panel, who was deemed to display anti-China bias in comments posted by the Italian on social media.

The second hearing, as required by the Swiss court, took place online last month overseen by a new panel of judges, who found that Sun "had acted recklessly" when he refused to let anti-doping officials leave his home with a sample of his blood.

The formal charges were refusing to give a sample and tampering with a doping control, according to the CAS ruling published on Tuesday.

Sun repeatedly protested his innocence during the protracted legal dispute.

He claimed that he refused to cooperate only after finding testers were operating without adequate ID proof and were taking photos of him without his permission during what was supposed to be a confidential procedure.

Despite the suspension, Chinese lawyer Zhang Qihuai, who represented Sun at the first hearing, described the swimmer as "the pride of China".

"The ruling by an international arbitration court is a valid one," Zhang said in a Weibo post on Wednesday. "But Sun's case has exposed the complexities involved in international legal cases dealing with foreign agencies, the weakness and defects of the domestic system to educate and protect athletes from the legal standpoint.

"I can definitely say that Sun did not violate the rules and there were no violations detected in the results (from doping tests)," said Zhang, founder of Beijing-based Lanpeng Law Firm.

Sun's last international appearance in the pool was at the 2019 FINA World Championships in South Korea, where he won the 200 and 400m free to take his long-course world-title haul to 11, making him one of the most decorated freestyle swimmers of all time.

During the meet, some foreign rivals, including Australia's Mack Horton who beat Sun to 400 gold at Rio 2016, snubbed the Chinese star by refusing to join him on the podium or shake his hand.