China's Sun Yang is awarded for best men's swimmer of the championships, at the Aquatics World Championships in Kazan, Russia, August 9, 2015. [Photo/Agencies]

KAZAN, Russia - No sooner had Sun Yang returned to the top of the podium with two victories at the world swimming championships than the Chinese star was enveloped in mystery.

Sun failed to show up for the 1,500-meter freestyle final Sunday night, creating chaos in the call room minutes before the race in which he was the two-time defending champion.

The 23-year-old swimmer later said a heart problem caused him to vanish.

Sun couldn't be found by the judges, and Lane 3 remained empty while Gregorio Paltrinieri of Italy won.

"Today I came here and during the warmup in the pool, I feel uncomfortable in the heart so I have to give up competition," Sun said through a translator at a news conference after the races ended. "I feel sorry because of that."

Earlier, FINA spokesman Pedro Adrega said Chinese team officials told him Sun felt a ``chest sensation'' during warmups.

"As there was not enough time to evaluate the situation or to check his medical condition, they decided to withdraw," he said.

Sun said he decided on his own not to swim the 1,500. He said he would return home Monday to arrange a medical exam of his heart.

Sun's rivals were shocked by the strange turn of events.

"The judges were going crazy yelling, 'Where's Sun Yang? Where's Sun Yang?'"Paltrinieri said. "It was insane."

Sun appeared at the end of the night to receive the best male swimmer award. He won the 400 and 800 freestyles in Kazan and was a strong favorite in the 1,500, which he won at the London Olympics. Sun finished second in the 200 free.

Brazil men's coach Alberto Silva told The Associated Press there was an incident in the warmup pool Sunday morning when one of his female swimmers was in the same lane as Sun, though Silva did not think it was the reason why Sun didn't compete.

"There was a discussion," said Silva, adding that no one was injured. "It's a short girl and a big guy. He went across (her)."

Silva said he complained to officials of world governing body FINA, and they spoke to a Chinese team official.

"(The Chinese) came to the Brazilians and said sorry. That's it," said Silva, who declined to identify the Brazilian swimmer.

Brazil team spokeswoman Eliana Alves told the AP there was contact between Sun and the Brazilian athlete "but it was not a fight."

Chinese team officials declined comment on the warmup pool incident.

"I have no comment because it's the morning program," Sun said.

Sun was back at the biggest meet before next year's Olympics after serving a three-month doping suspension last year for a banned stimulant. At the time, he said the positive test was caused by an ingredient in medication he was taking for a heart condition and he wasn't aware it was banned.

His punishment began immediately after he tested positive in May. However, Chinese officials kept the test quiet for six months and FINA also waited until late November to post the sanction on its website.

Sun got in trouble in November 2013 for driving without a license after the car he was operating was hit by a bus. Earlier that year he was formally censured by his training school after feuding with his coach and missing practice.

Paltrinieri took the lead from Cochrane at 500 meters and controlled the pace the rest of the way, touching in 14 minutes, 39.67 seconds. He finished second to Sun in the 800 and their rematch had been highly anticipated.

"I didn't know what to do or think," the Italian said. "It was really terrible without him."

The others in the call room wondered about Sun, too.

"I started saying, 'He's afraid of us and he's not coming,'" Paltrinieri said. "Then he really didn't show up."

Sun's absence threw off the Italian's race strategy.

"I've been training for two years on how to beat him, right down to the smallest details," Paltrinieri said. "I had to change everything."