Kazan - China's world record-holder and Olympic champion Sun Yang confirmed his shock withdrawal from Sunday's world championship 1 500m freestyle final was down to a heart problem.
The 23-year-old said he made the decision to pull out shortly before being called to the Kazan pool having felt unwell during the warm-up.
Sun, who also found himself Sunday at the centre of claims he was involved in a physical confrontation with a female Brazilian swimmer, explained he started feeling problems with his heart after retaining his 800m freestyle title on Wednesday.
The same problem in his chest returned while he warmed up for Sunday's 1 500m final.
"First of all, I am very sorry that I couldn't swim the 1 500m final," he said in a press conference.
"After the 800m competition, I didn't feel well in my heart. I felt uncomfortable in the warm-up tonight so I had to give up the idea of competing."
His withdrawal meant Sun missed out on repeating his 2013 sweep of the 400, 800 and 1 500m freestyle world titles having already won last Sunday's 400m final.
The Chinese giant courted controversy in 2014 after it emerged he had served a three-month doping ban for taking a banned stimulant.
Trimetazidine was in a prescription drug he sometimes took for heart palpitations.
Sun says it is the first time he has been affected like this and he wants to get to the bottom of the problem as quickly as possible.
He said he will be seeking medical advice as soon as he gets home.
"Yes, this is the first time I have felt uncomfortable with my heart in competition," he said as Italy's Gregorio Paltrinieri won the 1500m world title in his absence.
"I will go back to China tomorrow and will arrange to have a complete check of my heart as soon as possible."
Meanwhile, Sun gave a "no comment" to reports that he was involved in a physical altercation with Brazilian swimmer Larissa Oliveira in the warmup pool on Sunday morning.
Several swimmers confirmed there was a heated exchange while reports in Brazil's media claimed Sun elbowed Oliveira.
Brazilian officials have made a formal complaint to world governing body, FINA.
As Sun left it until the very last minute to withdraw from Sunday's race, his lane remained empty as FINA had no time to call up reserve Pal Joensen of the Faroe Islands.
"I made the decision by myself and didn't tell my coaches. As I said I am very sorry," said Sun who was names best male swimmer in Kazan by FINA.
"After the warm-up, and while I was preparing to take part in the competition, I realised I just couldn't compete. I didn't see what time it was."