Rio Olympics 2016: USA flag bearer Michael Phelps laments lack of 'clean sport'
Swimmer Michael Phelps says he does not believe he has ever participated in a "clean sport" after being chosen to carry the USA flag at the Rio Games.
Phelps, 31, is the most decorated Olympian of all time with 22 medals and the first American male swimmer to qualify for a fifth Games.
"We all want clean sports and everyone on the same playing field," he said.
"I don't think I've ever competed in a clean sport - but there's not much other than me I can control."
Phelps, who was chosen by a vote of fellow team members for Friday's opening ceremony, has won 18 Olympic gold medals.
"Something needs to change with all sports, not just swimming," he added.
"As athletes you want to compete on a level playing field - and how many Olympics have we had and been saying this at now? It's sad someone at the top can't get a handle on this."
Phelps retired after winning four gold medals at London 2012, before returning to the pool in 2014.
He was arrested for drink-driving later that year and banned from competitive swimming for six months.
Phelps returned to action in April 2015 at the Mesa Pro Series meeting in Arizona, winning the 100m butterfly, before claiming three gold medals at that year's US National Championships in San Antonio, Texas.
"I'm honoured to be chosen, proud to represent the US, and humbled by the significance of carrying the flag and all it stands for," he said.