Michael Phelps hails "unbelievable" Ahmed Hafnaoui after 18-year-old Tunisian's shock victory in the men's 400m freestyle

Michael Phelps hailed Ahmed Hafnaoui for his "unbelievable swim" on Sunday, saying the 18-year-old Tunisian's shock victory in the men's 400m freestyle was a great example of how swimming at the Tokyo Games is likely to have a series of wide open races.

Hafnaoui was the slowest qualifier but, from lane eight, held off fancied Australian Jack McLoughlin and American Kieran Smith for a huge upset.

"Unbelievable swim, I think he knocked almost five seconds off his pb (personal best), Phelps told reporters in Tokyo. "The difference between these Olympics and the past, in my opinion, is that every single person in the final has a chance of winning gold - it doesn't matter if you are in lane one, eight or four, everyone is close.

"Look at the 400IM (individual medley) - less than a second between first and eighth just to make the final. I just feel like the globe is starting to step up and swim faster and for me it's fun to watch. Its amazing to be able to see kids stand up and accomplish their goals."

Phelps said he was unsure whether one of the drivers of that levelling up was the impact of COVID-19, and consequent disappearance of so many races and training opportunities. "You can probably go both ways with that," he said.

"There are going to be some people who really stood up to the challenge, grabbed the bull by the horns and said 'let's go'. Then there others who will use the year maybe as an excuse. The people swimming well now looked at last year as an opportunity, as a chance to move forward.

"One thing I always tried to do was think 'what can I control?' so if you can figure out what you can control to the best of your ability you are giving yourself your best chance to accomplish your goals."

Phelps retired -- for the second time -- after the 2016 Rio Games with an astonishing 23 gold medals to his name and was speaking as part of a product sponsorship.

"It's challenging," the 36-year-old said of his "civilian" life. "It's unexpected, it's something you don't know. Swimming was all I knew for so long so transitioning out of the sport into the real world I'm going to have experiences that are eye-opening. But I think it's through the things you experience in your life that give you the knowledge to continue."