Justin Gatlin and Usain Bolt into World Championship 100m semis

Controversial American sprinter Justin Gatlin underlined his status as 100m gold medal favourite at the World Championships with the fastest time in the first round.

Gatlin, who has twice served doping suspensions, clocked 9.83 seconds in the sixth heat, albeit with the assistance of a 2.1m/s tailwind.

Reigning world and Olympic champion Usain Bolt won the seventh heat in 9.96 into the slightest headwind and grinned afterwards despite a slow start.

Bolt, 29, has been beset by a pelvic injury for much of the season, producing only two sub-10 second 100m performances all summer, and he looked to be working hard for the first 60 metres as he continues to search for the magic of old.

"It wasn't as great as I wanted it to be, but I wasn't expecting the first one to be great," said Bolt.

"I wasn't trying to run fast. I wanted to save as much energy as possible. I am in wonderful shape."

While Gatlin has been consistently running 9.8secs this summer, and arrived in Beijing on a 27-race unbeaten run stretching back two years, Bolt will go into Sunday's semi-finals and final hoping to hit a peak at exactly the right time.

Gatlin, 33, oozed the same confidence and easy form he has shown throughout the year, playing down his time, and once again he seems unaffected by the furore around his appearance here after those two drugs bans.

"I went out there and ran the first half of my race really well. I feel good," said the American.

"Bolt did the same thing in 2012. He raced slower in the first round, picked it up in the semis and crushed it in the final."

Also impressive was his young compatriot Trayvon Bromell, whose 9.91secs saw him win the fourth heat, while France's Jimmy Vicault and Jamaica's Asafa Powell also went under 9.96.

Tyson Gay, world champion in Osaka eight years ago, cruised through the second heat in 10.11.

But while British champion CJ Ujah came through as an automatic qualifier with a smooth 10.05 behind Bromell, there was disappointment for James Dasaolu.

Dasaolu has a personal best of 9.91 but got his heat totally wrong, shutting off too early before being swamped by his rivals.

The third Briton, Richard Kilty, made it through as a fastest loser and was again fortunate after easing off too early to drop down to fourth in the third heat in 10.12.