Usain Bolt denied suggestions he is retiring because he is afraid of 400 metre world record holder Wayde van Niekerk and bemoaned the fact he has not been able to compete against him.

Bolt is poised to call time on a glittering sprinting career in which he has won eight Olympic gold medals - he was stripped of a relay gold from the 2008 Games due to team-mate Nesta Carter's doping offences - having stated he will retire after the World Championships next month.

He will only run the 100m and the 4x100m relay at the Worlds in London, with Van Niekerk, who smashed Michael Johnson's 400m world record at the Rio Olympics last year, set to take part in the 200m event that Bolt has also dominated.

Asked at a media conference ahead of Friday's Diamond League meeting in Monaco if he feared Van Niekerk, Bolt replied: "Definitely not. You guys know me over the years, I never back down from a challenge.

"I think that's the most disappointing thing in my career now that he came along at this late stage, that I didn't get to compete against him.

"I think he's one of the best - hands down - right now and he's really confident and he really works hard. 

"I've seen him train because he has come to Jamaica already and I've seen him at the level. [But] I'm never afraid, I live for competition. But it's too late now, I'm at the end of my career now so, we'll never know what could've happened."

Quizzed on why he is retiring at age 30, Bolt said: "I've done everything that I possibly can in the sport.

"I've always told you guys that I've dominated over the years. I asked Michael Johnson - because I was surprised when I heard he retired - I asked him, 'why did you retire when you were dominating so much?' And he said, 'I've done everything I wanted to do, I've accomplished all my goals.' And that's how I feel at this point so I decided it's just time."