Then in 2015, he became famous not only in Asia but also in the world. Ning won the men's 100m freestyle at the FINA World Championships in Kazan, Russia, on Aug 6, becoming the first Asian champion at the event. The champion was described as a new "national idol" in many media outlets and became the sweetheart of many Chinese women.

That was the high point of Ning's career. Posts containing the hashtag #Ning Zetao's girlfriend were read 270 million times on Sina Weibo, China's Twitter-like social networking site, as 185,000 Chinese internet users engaged in the conversation. Many female Weibo users even photoshopped Ning Zetao's photos on their selfies and pretended to be Ning's girlfriend.

After rising to stardom when he won the men's 100-meter freestyle championships, he became the hot favorite ahead of the Rio Olympic Games. However, he failed to make the final of his favorite event, finishing 12th in the men's 100m free and 30th in the 50m free.