China's star swimmer Sun Yang has vowed to defend his titles at the swimming world championships in July in Kazan, Russia.

Sun, who swept the 400m, 800m and 1,500m freestyle titles at the 2013 Barcelona Worlds, has booked a quota for the 200m, 400m, 800m and 1,500m at the 2015 Kazan Worlds after winning all the events at the National Swimming Championships, which served as a qualifier for the Worlds and ended late Thursday.

"I hope I can defend my world titles in Kazan," said Sun, who served out a three-month doping ban last year. "The 400m free will be the first event I take part in at Kazan, and I hope it will be a good start."

The London Olympic 400m and 1,500m winner went through a difficult time after the news broke by the end of last year that he had served out a three-month ban in 2014 after testing positive for the banned stimulant trimetazidine on May 17.

Sun proved his innocence with sufficient evidence in July's hearing that he was not aware that the substance was prohibited and took the prescription drug Vasorel to treat a heart condition.

Australian Denis Cotterell was not allowed to coach Sun any longer as a result of the new policy implemented by Swimming Australia. Sun was thus forced to train at other clubs in Australia, and according to Zhang Yadong, Sun's coach, the 23-year-old prodigy worked hard during the 13-week winter training and lost some weight.

Another Australian coach, Brain King, now works with Sun. King was Cotterell's assistant at the Miami Club where Sun used to train. And basketball star Stephon Marbury's physical trainer Chris Hoffman has also joined Sun's team.

"Sun had been in a poor mental state," said Zhang. "He didn't want to talk and go outside, and didn't want to meet anyone. It was not easy for him."

The Nationals was the first competition for Sun this year. Though he won four titles at the Nationals, Sun didn't produce a world fastest time. He only ranked sixth in his signature 1,500m free, and was fourth in 400m, second in 200m and 800m.

"It's not reasonable to expect him to win all the events at the Worlds," said Zhang. "We have not decided to take part in which events yet. Our aim is the Rio Olympic Games, and the Kazan Worlds is only a warm-up."

Yet Sun seemed positive towards the worlds himself.

"I still have time," he said. "I think my results are OK this time."