China's former Olympic champion swimmer Sun Yang has reiterated his ambition of coming back to compete as strong as ever after his doping ban of over four years finished on Tuesday.

Sun, a former three-time freestyle Olympic champion, technically regained his eligibility to compete in an official race again on May 28 after completing a suspension of four years and three months, issued by the Court of Arbitration for Sport, that took effect from Feb 28, 2020.

Sun, who turned 32 in December, said during an exclusive interview with ThePaper.cn that he's never given up on his athletic career throughout the tough years and he's ready to come back as a competitive swimmer again.

"I'd decided four years ago (when the suspension was imposed) that I would not quit. Now I just hope to be back in the pool and to step back on the starting blocks again as soon as possible," Sun said during the interview.

Sun's controversial doping case dated 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 World Aquatics (then known as 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 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 in May 2021 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.

Sun had 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.

The Lausanne-based CAS announced in June 2021 to reduce the initial eight-year ban into four years and three months, citing rule amendments that allow flexibility to assess the "entirety of the circumstances".

"At the beginning (of the suspension), I suffered a lot emotionally and mentally, feeling nothing but darkness. My parents almost tried to satisfy every one of my demands at that moment, worrying that I would suffer a mental problem," Sun confessed.

"Then slowly and gradually, I managed to come out of the shadows, learned to appreciate and enjoy other things in my life, got married, went back to university to continue my academic studies.

"Looking back now, I feel I've gained a lot more than what I've lost during the four years.

"Now I've left it all behind, made peace with what happened in the past and am ready to give it a go again," said Sun, who won two gold medals (400m and 1,500m free) at London 2012, followed by a 200m gold at Rio 2016.