Superstar Sun Yang broke down in tears after dedicating his latest gold medal to his late friend Kenneth To.

Hong Kong swimmer To died last week after falling ill in the locker room while training in Florida.

"I wanted to have a great performance today because I did not swim only for myself. I wanted to try my best to honor my friend Kenneth," said an emotional Sun after winning the 400m freestyle in a world-leading 3 minutes, 43.73 seconds at the national swimming championships in Qingdao, Shandong province, on Sunday.

"When I heard the news, I could not hold back the tears. I could not believe it. I still remember the meal we had together in Hangzhou."

To was a 4x100 teammate of Sun at the 2017 National Games and in December competed at the world short-course championships in Hangzhou.

The 26-year-old was training at the University of Florida when he became ill and died in hospital.

The cause of death was not immediately clear.

"Being a professional swimmer is not easy. We all face huge pressure physically," said Sun.

"More and more people are paying attention to swimming, which can put huge mental pressure on us. I just hope every athlete can stay healthy."

Sun also reflected on the perils of swimming's grueling training requirements in a touching tribute to To last week.

"Being a professional athlete is a dangerous profession. Long-term overload training is the path we choose to chase our dreams," Sun wrote on Weibo. "You stopped on the road of chasing the dream, but we will charge forward fearlessly. Thank you for everything you've done for swimming."

Sun's outpouring of emotion has struck a chord with fans, with his post-race interview becoming one of the hottest topics on Chinese social media on Sunday night. An hour after the race, related topics were viewed over 55 million times, while countless fans sent Sun their condolences and best wishes.

Despite his gold medal heroics at the nationals, Olympic and world champion Sun said he is still feeling the effects of a tough winter training camp in Singapore.

"After a period of extremely intensive training, now I've finally returned to competition," said Sun. "But I'm actually not in perfect condition because we were really exhausted during winter training. Many of my teammates were sick.

"For me, being here healthy and finishing the competition smoothly after the intensive training are really good signs that I'm on track.

"I set a season-best time even in such difficult conditions, which is evidence that I can bring more honor back to our nation at the Gwangju world championships (in July in South Korea)."

Sun's coach, Zhu Zhigen, was also looking at the bigger picture and said gold in Qingdao was a bonus.

"For Sun, the main thing is being relaxed and avoiding shouldering a heavy burden," said Zhu.

"Athletes don't have to be perfect at every competition. He just needs to hit the targets we set. That's good enough."

As well as the 400, Sun signed up for the 200, 800 and 1500 freestyle competitions at the nationals.

Finishing all four events in eight days is a major test of his endurance, but one which the three-time Olympic champion is relishing as he ups his preparations for the 2020 Tokyo Olympics.

"I need to maintain a good performance to let the fans see I'm capable of lasting the pace of a long event," said the 27-year-old.

"I can still compete with others round after round.

"It's tough. It's tiring and difficult for me. I need medical treatment to recover every day. But I hope by next year all of the effort will be worthwhile."