Eight-time Olympic champion Usain Bolt described his performance as "very bad" as he won his 100m heat at the World Championships in 10.07 seconds.
The Jamaican, who is retiring after the event in London, beat Great Britain's James Dasaolu into second (10.13) but was frustrated with his start.
"I'm not fond of these blocks," said Bolt. "I have to get this together."
The USA's Justin Gatlin, twice suspended for doping, was booed when he came out to race but eased through.
Britons Reece Prescod (10.03) and Chijindu Ujah (10.07) took top-three spots to reach Saturday's semi-finals.
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Lifetime best and block frustration
Prescod's mark was a lifetime best and earned him third place in a heat where Jamaica's Julian Forte produced the fastest time of the night to win in 9.99 seconds.
The British champion said: "My first World Championships I come out with a PB in the first round - I can't really complain.
"Being in front of a home crowd it's great, this is what athletics is all about - I loved it."
Bolt in contrast looked like he needed to work hard through the middle of the race before tapering down and shaking his head after crossing the line.
The 11-time world champion has only run under 10 seconds once this year and in what was his fourth race of 2017 over the distance he has made his own for a decade, he pointed to his starting blocks as a source of frustration.
"It's shaky because when I did the warm up it [the blocks] pushed back," he told BBC Sport. "It's not what I am used to."
Analysis - Do others believe?
Former Great Britain sprinter Darren Campbell on BBC Radio 5 live: "This is what we mean by athletes believing they can beat Usain Bolt.
"Two or three in this field have run quicker than him. Where he would have liked to step it up a bit and look easy, he had to work and then started looking across. He expected more. But we know he loves the rounds.
"He tends to make mistakes in the early rounds but I'm sure he wont make the same in the semi-finals."
Four-time Olympic champion Michael Johnson on BBC One: "The race didn't look like normal Usain Bolt. He obviously felt something in the blocks he didn't like and it affected him in the race. He normally doesn't race in the first round and just runs what he needs to run, but he was actually competing and it just looked a little weird."
The men seeking Bolt's crown
American champion Christian Coleman - the fastest man in the world this year - looked comfortable, easing down to a 10.01 victory.
His win came in the first of six heats, with three men qualifying from each as well as six fastest losers, cutting a 48-man field down to 24 athletes for the semi-finals.
Jamaican Yohan Blake - the second fastest man in history behind Bolt - looked slightly laboured in placing second in 10.13 behind 18-year-old Abdul Hakim Sani Brown of Japan (10.05).
American Justin Gatlin cruised to a 10.05-second victory but it was Jamaica's Forte who set the marker on the night as the only man to break 10 seconds.
He edged out Ivory Coast's Ben Youssef Meite and Prescod in a race where Akani Simbine disappointed in coming fourth. The South African has run sub-10 on eight occasions this season but required a fastest-loser spot to progress.