Usain Bolt admits he cannot be sure the Olympic 100m final in Rio will be a drug-free event. The reigning champion and world record-holder claims athletics is slowly "weeding" dopers out of the sport, but conceded the problem looms large in Brazil.

The build-up to the Games has been dominated by the doping scandal involving Russia, while two of Bolt's biggest rivals for gold medals, Americans Justin Gatlin and LaShawn Merritt, have previously served drug bans. However, Bolt has urged fans to keep faith with athletics.

"People should have faith," said Bolt, according to the BBC. "We're weeding out the bad ones. We're going in the right direction."

Despite this, the six-time Olympic champion later added that "nothing is guaranteed" when asked whether the sprint races at the Games would be drug-free. Bolt – who is seeking to defend his 100m and 200m titles in Rio – also revealed which of his races in making him most anxious.

"The 100m is never stressful," Bolt said. "For some reason, when I get to the 200m, I'm always a little bit nervous."

Bolt, 29, is hoping to secure a third successive Olympic treble in Rio after triumphing in the 100m, 200m and 4x100m relay titles in Beijing and London in 2008 and 2012 respectively. But regardless of his results in Brazil, Bolt has reaffirmed that this will be his last-ever Games, as he plans to retire from athletics following the 2017 World Championships.