World record holder Usain Bolt cruised through the 100m heats at the World Championships, but Briton James Dasaolu only made it as a fastest loser.
Dasaolu, 25, became the second-quickest Briton of all time with 9.91 seconds last month, but could only manage 10.20 as he finished fourth in his heat.
Bolt had no such trouble, easing to victory in the final heat in 10.07.
American Mike Rodgers was quickest in 9.98, while Britons Dwain Chambers and Harry Aikines-Aryeetey also progressed.
Dasaolu's involvement almost finished at the earliest stage as he appeared to ease off approaching the line and was edged out of the three automatic qualifying places.
"I've learnt a lesson," the Londoner told BBC Sport. "I don't think I got off to a great start. I'll just use the round and try to build on this tomorrow.
"I haven't been able to do much training in the run-up to this and I knew I'd be a bit rusty but hopefully I can step my game up tomorrow."
Aikines-Aryeetey and Chambers were both third in their heats and, after a nervous wait, Dasaolu learned he had qualified by 0.01 seconds.
"It was tough," said Chambers.
"The first round is always nerve-wracking because you want to get through safely and it was a tough heat with the likes of Gatlin and (Keston) Bledman - sub 10-second runners.
"Medals are up for grabs here but you've got to take each round one step at a time, and I know for me I've got to rely on my experience, most definitely."
Rodgers and compatriot Justin Gatlin, with 9.99, were the only men to dip under 10 seconds, but Bolt barely broke sweat as he dominated his heat.
The Jamaican is looking to regain the world title he lost in Daegu two years ago when he false started in the final, and then had to watch as compatriot Yohan Blake took the title.
There was an echo of that moment in Moscow when the Cayman Islands' Kemar Hyman was disqualified for a false start, but Bolt insisted: "I wasn't really worried. I was listening for the gun, so that was good."
With Blake injured and American star Tyson Gay absent because of a doping scandal, Bolt is an even firmer favourite to win gold on Sunday.