Allyson Felix has admitted concerns about potential doping at Rio 2016, but feels appropriate steps are being taken to eradicate the issue from athletics.
Felix qualified for her fourth Games this month but missed out on the chance to attempt a double in the 200 and 400 metres after finishing fourth in the shorter distance at the United States Olympic trials.
Regardless of the events she is competing in, the 30-year-old is unsure a level playing field can be guaranteed in Brazil.
"The number of scandals we see now just shows the real issues that are there in sports," Felix said.
"I would say that my confidence isn't high to think that there are not real issues.
"I would hope that the playing field is more level. It is very hard to deal with as a clean athlete.
"You are going to the biggest race of your life and you hope that that is a clean race, but can you say that you are 100 per cent sure?
"I don't think that you can say that."
Russian track and field athletes are currently banned from international competition following allegations of "state-sponsored doping".
In May, the International Olympic Committee announced 31 athletes failed drug tests following reanalysis of samples from Beijing 2008, while there were 23 positive results following London 2012 re-tests.
Felix believes adequate steps are being taken to try and clean up athletics.
"It's hard ... but I hope we are moving in the right direction. I think that exposing athletes that are happening now is a step in the right direction," she said.