Usain Bolt's decision to call time on his glittering career will dominate the IAAF World Championships in London that starts Friday.

The Jamaican sprinter, an eight-time Olympic gold medallist with 11 world golds to his name and world record holder in the 100 and 200m, will race just the 100m and 4x100m relay in the British capital.

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The 30-year-old will leave a huge gap for track and field to fill, Bolt's startling on-track achievements having been accompanied by a charismatic yet humble personality that has never ceased to draw attention for all the right reasons.

In the past few seasons, that has often come with athletics at its lowest ebb thanks to doping crises and linked corruption at the very heart of the sport's presiding body.

"He is the best sprinter of all time," was IAAF president Sebastian Coe's blunt assessment of Bolt.

"Usain Bolt is a genius. I can't think, other than Muhammad Ali, of anybody that has so had an impact inside or beyond their sport.

"You can have the Friday-night-in-the-pub conversations about who is best footballer or tennis player, but there is no argument about this guy in sprinting."

Coe added: "What we will miss is the personality. We do want athletes with personality. It's nice to have someone who has a view and fills the room and fills a stadium."

Bolt insisted he would go into the defence of his 100m crown as an underdog.

"That's what I keep reading and what my team keeps telling me, so I've got to prove myself again," he said in a stark warning to potential rivals.

He added: "I'm comfortable saying I'm a legend because I've proved myself. Off the track, I'm as simple as can be."

With Bolt nearing the exit, much talk has focused on who could likely fill his large spikes.

One name pops up continually: that of South African Wayde van Niekerk, the current world and Olympic 400m champion who destroyed American Michael Johnson's world record when winning at the 2016 Rio Games.

"Wayde's doing a pretty good job," Bolt said, Van Niekerk having also broken Johnson's world record of the rarely-run 300m at Ostrava in June.

"He's now going to run the 200m, that's going to be exciting. He's a really cool person." 

Van Niekerk's ambitious attempt at the 200-400m double, previously achieved only once – by Johnson in 1996, makes for six days of intense competition.

"Usain has been a massive inspiration," said the 25- year-old Van Niekerk.

"But I've still got quite a long way to go before I even get close to the heights that Usain has reached."

Another athlete sure to be a crowd pleaser will be Briton Mo Farah, who is seeking a 10th consecutive global title stretching back to 2011 and incorporating the 5000/10,000m doubles at both the London and Rio Olympics.

Farah, 34, honed his speed with victory over the 3000m at the London Diamond League, just after his medical records were hacked and leaked by the Fancy Bears group.

The Somali-born Londoner played down off-track distractions, saying he would give it all in his last track season before turning to marathon running.

"I love being on the podium, hearing the anthem and making people proud to be British. I love what I do and I'm good at what I do — you can't worry about other stuff," he said.

Kenya, already hit by the absence of Olympic 400m hurdles champion Nicholas Bett, suffered a further blow when David Rudisha was a late withdrawal.

The 28-year-old, who ran an astonishing world record time of 1min 40.91sec to take gold at the 2012 London Olympics, said he was out "due to a quad muscle strain".

Doping-tainted Russia remains banned from international competition, although Coe confirmed that 19 Russian athletes had met the necessary criteria so as to compete in London as neutrals.