Usain Bolt has described his World Championships 100m final victory over Justin Gatlin as the "hardest race" of his career.

The Jamaican, 29, lived up to his billing as the saviour of athletics as he held off the challenge of the two-time drug cheat to win in 9.79 seconds.

Pre-race favourite Gatlin had to settle for silver in Beijing.

"Coming back from injury I've had a lot of doubters, it's been tough," Bolt said after winning a ninth world title.

He added: "For me to come to the championships and defend my title is a good feeling.

"I definitely think this was my hardest race. I've been through a lot this season."

Usain Bolt crosses the finish line.

Bolt, back at the scene of his first triple Olympic triumph in 2008, called his Munich-based doctor, Hans-Wilhelm Muller-Wohlfahrt, "a lifesaver".

He also insisted he did not feel the pressure to win for his sport, only to continue his own global domination.

And that looked on course to end after the semi-finals when Gatlin clocked 9.77secs, while Bolt almost tripped out of the blocks and had to fight all the way to the line to snatch the win.

"After the semi-finals my coach [Glen Mills] said, 'You are thinking about it too much. There's too much on your mind, all you have to do is remember is that you've done this a million times so just go out there and relax'. That's what I did."

There was a surreal moment before the final when 'the world's fastest piano player' was brought out to play a tune for 9.58 seconds – Bolt's world-record time.

The Jamaican's reaction was a shrug of bemusement as he showed no sign of nerves on the start line.

Instead, Gatlin was the one to feel the pressure. And the 33-year-old said he "gave away" victory.

"I stumbled in the last five metres, my arms were a little flailing," he said. "You have to come out and run and over the last five metres. It wasn't my day.

"Anyone who goes to the line to go against Usain has to be ready to go to work. In those five metres I let things get away from me. It cost me the race.

"I leaned a little too far forward and I got a little off balance."

Americans Trayvon Bromell and Canada's Andre de Grasse, who are both 20, shared bronze in 9.92secs.

Meanwhile Gatlin will have an opportunity for revenge when he and Bolt go head to head over 200m, with the final taking place on Friday.