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."
Connor Jaeger of the United States finished second in 14:41.20. Cochrane, who was under world-record pace through 400 meters, was third in 14:51.08. He earned his fourth straight medal in the 1,500, equaling Australian great Grant Hackett.
"We would have liked the best field possible but it was still a pretty hard race," Cochrane said.
The United States received the best team trophy for the eight-day meet, winning 18 medals, including seven golds.
Katinka Hosszu of Hungary made a run at the world record in the women's 400 individual medley before falling short on the last freestyle lap. She won in 4:30.39, after being 3.81 under world-record pace after five laps.
Hosszu, nicknamed "Iron Lady" for her relentless event schedule, also won the 200 IM in world-record time.
The United States won the men's 4x100 medley relay two years after touching first and then being disqualified when Kevin Cordes dived in too soon on the breaststroke leg.
Ryan Murphy, Cordes, Tom Shields and Nathan Adrian won in 3:29.93.
Australia was second and France third at Kazan Arena, where Russian Prime Minister Dmitry Medvedev was in attendance.
The women's 4x100 medley relay was claimed by China's team of Fu Yuanhui, Shi Jinglin, Lu Ying and Shen Duo in 3:54.41. Sweden was second and Australia finished third.
Japan's Daiya Seto cruised to a 1.40-second victory for his second consecutive title in the 400 individual medley.
Seto led all the way and won in 4:08.50 — 0.19 faster than he swam two years ago in Barcelona.
Bronte Campbell of Australia won the 50 freestyle to go with her title in the 100 free. She touched in 24.12.
Olympic champion Ranomi Kromowidjojo of the Netherlands was 0.10 behind in giving up the title she won in Barcelona. Sarah Sjostrom of Sweden took third in 24.31.
Campbell's older sister, Cate, was fourth.
In a pair of non-Olympic events, Jennie Johansson of Sweden earned a surprising win against a talented field in the 50 breaststroke, while Camille Lacourt of France won the men's 50 backstroke.
This story has been corrected to change attribution of quote in 10th paragraph to Paltrinieri.
AP Sports Writer Andrew Dampf and Associated Press writer James Ellingworth contributed to this report.