Sun Yang of China competes in the men's 1500m freestyle heats at the Aquatics World Championships in Kazan, Russia, Aug 8, 2015. [Photo/Agencies]

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.