BEIJING, July 20 -- Chinese Olympic and world swimming champion Sun Yang is back to shape as he builds up for the world championships starting this week.

The 23-year-old Sun, who claimed three gold medals at the 2013 Barcelona worlds, is expected to compete in four events, 200m, 400m, 800m and 1,500m freestyle at the Kazan World Championships from July 24 - August 9.

Sun's coach Zhang Yadong believed his protege is close to his top form.

"Sun is almost back to his peak. Judging from his training, he is about 90 or 95 percent of his peak form," said Zhang on Sunday.

Sun, however, is aware that it will be a daunting task to repeat his feat in the 2013 worlds.

"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," he said.

The swimmer, entangled in brawls with former coach Zhu Zhigen and a drunk driving scandal in the past few years, kept low key about his goals at Kazan.

"It will be a huge challenge for me. I'd rather keep the goal in my mind than speak it out," he said.

Sun is favored to win the men's 200m free title, as Olympic champion Yannick Agnel and Asian champion Kosuke Hagino both have decided to skip the worlds due to injuries. Sun's old rival, South Korean Park Tae-Hwan, will also miss the worlds over a doping ban.

Also on Sunday, a 51-athlete roster was revealed as Olympic champions Sun and Ye Shiwen spearhead the team but another Olympic champion Jiao Liuyang and world champion Liu Zige will miss the trip.

Liu, reigning 200m butterfly world champion, has been in a low tide since April and failed to make the cut while Jiao has been suffering from a stomach disease and is in treatment now.

The Chinese swimming team will leave for Kazan on July 27.