Usain Bolt admits he cannot be completely sure that his sprint rivals at the Rio Olympics are all clean.
The Jamaican is aiming to complete the 'triple triple' of Olympic sprint titles in Brazil by retaining his 100, 200, and 4x100 metres relay titles.
The build-up to the Games has been dominated by the Russian doping scandal, while the world number ones over 100m and 200m - Americans Justin Gatlin and LaShawn Merritt - have both served drug bans.
"In life nothing is guaranteed," the 29-year-old said when asked if he could be certain the sprint races in Rio would be drug-free.
"For me going out there I never worry about that. I just go out there and compete.
"I think we're going in the right direction, I must say. I think we're weeding out the bad ones. We have to go through a rough time before we get to the good times."
[[From earlier] Usain Bolt talks nerves ahead of #Rio2016.RDT— EWN Reporter (@ewnreporter) August 8, 2016
Having already taken a full set of golds home from Beijing in 2008 and London four years later, the world's fastest man is looking to bring his haul of Olympic titles to nine.
He declared after retaining his 100m and 200m crowns in London that he had become a "living legend".
Doing so again would elevate him to an all-new level of greatness.
"As a young kid you grow up looking forward to the big Games," he said. "Championships are what matters. This is what I do, I enjoy doing it.
"I've got to prove myself over and over again."
Queues to get into the press conference at the Cidade das Artes, the home of the Brazilian Symphony Orchestra and the largest theatre in South America, were building more than two hours before it was due to start.
And when it got under way there were fewer empty seats than at many of the events so far.
No one in athletics - and few in sport - draw crowds like Bolt and how to fill the void he will leave when he is gone is turning into one of track and field's biggest dilemmas.
And his press conferences are never ordinary.
From a Norwegian man declining to ask a question - instead proclaiming his love for Bolt before rapping a song in his honour and then blowing him kisses - to the scantily-clad samba dancers who came on at the end for him to dance with, they, like him, are a little bit different.
"I like to entertain, because that's what people come out to see," he added. "I try to entertain and make it different. That's my personality."
Bolt, who has stated his intention to retire following next year's World Championships in London, overcame a fitness scare to reach the Olympics after a hamstring strain forced him to pull out of the Jamaican trials.
He blew away any injury concerns on his last appearance, however, clocking 19.89 seconds over 200m in London and insisted he was in better condition this year than when he was at last year's World Championships in Beijing.
It was an ominous warning to his rivals - he still left China with three gold medals.
Not that his coach, Glen Mills, is letting him get carried away.
"He said 'that's one of the worst races you've ever run, the corner was awful', he just went on till I got depressed," said Bolt.
Bolt first takes to the striking blue track at the Olympic Stadium in the 100m heats on Saturday.
It is seven years now since Bolt ran his world records - 9.58 for 100m and 19.19 for 200m - in Berlin, also, coincidentally, on a blue track.
And it is the 200m mark he wants to break more than anything.
He added: "I really want it, I really, really want that one. I've always wanted to run sub-19, so I'm really focused on that.
"I'm more nervous over 200m than anything else. For the 100m it's never really that stressful. I know where I'm weakest and I know when I'm strongest. With the 200m I'm always nervous, from the heats all the way to the end."
Gatlin, who has served two doping bans, has run the two fastest times over 100m this year and is likely to be Bolt's biggest obstacle to success in Rio, again pitching the Jamaican into the role of possible 'saviour' of a sport plagued by doping scandals.
Bolt has already accused the American of "disrespect" in the build-up for claiming the six-time Olympic champion got preferential treatment in being allowed to miss his trials.
Their showdowns on the track are set to be spicy. Bolt's best time over 100m this year is 9.88, ranking him joint-fourth.
Over the longer distance he is ranked fifth, with Merritt, another American back from a doping ban and better known as a 400m specialist, number one with 19.74. They will both know that the Olympics is Bolt's stage, though.