Rio de Janeiro, Aug 9 (IANS): Six-time Olympic gold medal winning sprinter Usain Bolt has admitted there are no guarantees he will be defending his sprint titles against a completely clean field here.

In a bid for a golden farewell to the Games, the track-and-field's biggest star will seek a third consecutive win in the 100metre, 200m and 4x100m relay at the Rio Games, although his new challenge is to beat the 200m world record that he himself set at 19.19 seconds.

The build-up to Rio has been dominated by the Russian doping scandal, while Bolt's rivals Justin Gatlin and LaShawn Merritt have both served drug bans.

“In life nothing is guaranteed. For me going out there I never worry about that. I just go out there and compete,” Bolt was quoted as saying by Sky Sports when asked whether he could be certain of a drug-free sprint contest on Monday.

"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,” the Jamaican added.

Bolt, 29, again confirmed he is competing in his final Olympics.

Bolt clocked 19.89 seconds in 200m at the Anniversary Games in London in July in his first appearance since pulling out of the Jamaican trials.

He was picked for the Olympics after getting a medical exemption, leading to comments from Gatlin and others.

Gatlin, considered his main 100m rival, was among those who earlier claimed the Jamaican - who has struggled with a hamstring injury - got preferential treatment he would not receive were he an American.