After solidifying his freestyle supremacy at the ongoing world championships, China's Sun Yang is bracing for a tougher battle to navigate a vortex of alleged doping violations.

What was supposed to be a historic night for Sun following his fourth 400m freestyle world title on Sunday ended up a farce after his Australian rival Mack Horton, runner-up in the final at the FINA World Championships in Gwangju, South Korea, refused to share the podium with Sun and bronze medalist Gabriele Detti of Italy, instead standing behind the platform during the playing of China's national anthem.

Sun became the only swimmer to win the 400m free at the worlds for a fourth straight time after clocking 3:42.44 to finish 0.73 seconds ahead of Horton, bringing his collection of world titles to 10 across four freestyle disciplines from 200 to 1,500m.

However, the celebratory mood was spoiled by Horton's controversial behavior, which he said was a protest against Sun for the Chinese star's alleged involvement in doping violations.

Boasting exceptional endurance and stroke efficiency honed by hard training programs, Sun has dominated the freestyle discipline in recent years, highlighted by three gold medals at two Olympic Games (London 2012 and Rio de Janeiro 2016).

His battle out of the pool, though, seems a tougher one as he struggles to restore a reputation tarnished by reckless handling of doping control issues in recent years.

Although allowed to compete in Gwangju, Sun is facing a Court of Arbitration for Sport (CAS) hearing in September over reports that he rejected a doping test last fall while questioning the testers' legitimacy.

According to Sun's attorney, Zhang Qihuai of Beijing Lanpeng law firm, two of the three testers, who failed to produce adequate documentation from the World Anti-Doping Agency, were later found to be untrained acquaintances of the main testing official.