Usain Bolt once again produced his best when it mattered most to retain his 100 metres world title on Sunday and reassert his status as the fastest man on the planet.

The 29-year-old Jamaican surged past the in-form American Justin Gatlin over the last 30 metres and crossed the line in 9.79 seconds, his fastest run of the year.

Gatlin finished second in 9.80 to take silver, while Andre de Grasse of Canada and American Trayvon Bromell finished together in third place in a time of 9.92.

They will both be awarded bronze medals after they ran identical times down to a 1000th of a second.

Bolt, the reigning Olympic champion and world record holder, has now not been beaten in the 100m or 200m in six major global championships going back to 2007.

"This definitely means a lot because I’ve been struggling all season," Bolt, who has battled for form in an injury-disrupted season, said in a trackside interview.

"It took me a while to figure out what was the problem and get it together. So I’m just happy."

He was disqualified from the shorter race at the Deagu world championships in 2011, however, and flirted with a similar mishap when he stumbled out of the blocks in the semi-finals earlier on Sunday.

There were no mistakes when it came to final, though, and, back at the stadium where he first rocketed into the sporting stratosphere at the 2008 Beijing Olympics, he powered down lane five to edge Gatlin.

The 33-year-old American, a world and Olympic champion before the rise of Bolt and his four-year doping ban, will perhaps regret dipping for the line quite so far out, a move which disrupted his rhythm and reduced his speed.

The victory for Bolt will be an undoubted boost for a sport which has spent three weeks locked in a public relations crisis after allegations of widespread doping among athletes.

"I understand why (people thought it was important for me to win)," Bolt added. "But as I said, I wanted to do it for myself. It was a big deal."

Bolt will go for successive world championship sprint sweeps in Beijing with the 200 metres final scheduled for Thursday.

"I'm amazed," said American track great Michael Johnson.

"This is Usain Bolt's best race ever. He knew it was a completely different set of circumstances for him. He'll be most proud of this win without a doubt.

"It's not about technique, it's about running for your life and he's got the talent to be able to do that."