At London in 2012, Sun became the first Chinese man to win an Olympic swimming gold medal when he took the 400 metres and 1,500m freestyle double.
In the three years since, however, his life has hit troubled waters.
In 2013 he found himself in hot water with Chinese swimming officials for missing training sessions and was suspended from engaging in any commercial activities, which had flooded in after his Olympic success.
He then fell foul of the law and spent a week in jail for crashing a car that he had driven without a licence. China’s swimming authorities responded by slapping a blanket suspension on him, banning him from all training and competition.
Last year it emerged Sun had secretly served a three-month ban after testing positive for a banned stimulant.
The China Anti-Doping Agency could have imposed a longer ban but opted for a lenient punishment because Sun had been given medication by a doctor to treat a heart issue and was unaware it had been added to the World Anti-Doping Agency’s banned list.
Through it all, Sun remained unfazed.
And in contrast to most of China’s top swimmers who try to avoid the limelight, his performances in the pool remained at the same high level.
At the 2013 world championships in Barcelona, the 1.98 metre (6ft 6in) tall Sun scooped up three gold medals (400m, 800m and 1,500m freestyle) to join Australia’s Grant Hackett as the only men to win the long-distance treble.
The following year, with news of his three-month ban still secret, he openly taunted his South Korean rival Park Tae-hwan at the Asian Games, but backed up his trash-talking on the blocks by winning another three gold medals.
The stigma of serving a doping ban has done nothing to dampen his enthusiasm as he sets his sights on next year’s Olympics in Rio, opting to tackle four individual events in Kazan in his last big dress rehearsal.
In addition to the three events he won in Barcelona, the 23-year-old has added the 200m freestyle to his busy schedule, a race he is now looming as one of the favourites in the absence of American Michael Phelps, French Olympic champion Yannick Agnel, Japan’s Asian champion Kosuke Hagino and Korean Park.
“When I was little, I watched Michael Phelps compete in five or six events at a worlds, so I want to try it as well,” Sun recently told China’s official Xinhua news agency.
“It will be a huge challenge for me. I’d rather keep the goal in my mind than speak it out.”
Although he did not need to swim at his best to qualify for the 51-member Chinese team picked to compete at Kazan, Sun is still ranked second in the world in two events, and in the top eight in all his races, and his coach Zhang Yadong said he would be in top shape in Russia.
“Sun is almost back to his peak,” Xhang said. “Judging from his training, he is about 90 or 95 percent of his peak form.”
The eight-day swimming programme in Kazan begins on Aug. 2.
(Editing by Peter Rutherford)