Usain Bolt admitted he has to "get this together" after failing to impress despite beginning his IAAF World Championships campaign with victory in his 100-metre heat on Friday.
The Jamaican icon, an eight-time Olympic champion with 11 world titles, will hang up his spikes following the championships and clocked a time of 10.07 seconds to advance to Saturday's semifinals.
Bolt bowed to the fans before setting off in the sixth and final heat, but only crossed the line first — seemingly leaving plenty in the tank — after starting sluggishly.
It was the eighth-fastest time of the night and the 30-year-old was clearly unimpressed, shaking his head after looking at the replay on the big screen.
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Bolt was critical of the starting blocks being used at London Stadium, but acknowledged he needs to produce more efficient starts.
"I was very bad. I stumbled a little bit, coming out my blocks. I'm not really fond of these blocks. I think these are the worst blocks I've ever experienced and it was just not a smooth start," Bolt told the BBC after the race.
"I have to get this together. I have to get started because I can't keep doing this.
"It's shaky — when I did my warm-up and pushed back it [the starting block] fell back. It's just not what I'm used to."
Julian Forte was the fastest qualifier and the only man to dip below 10 seconds, while promising United States sprinter Christian Coleman also impressed. Andre De Grasse, who was expected to challenge Bolt in this meet, withdrew earlier in the week after suffering a muscle strain.
Coleman, the 21-year-old world-leader after a 9.82 seconds run in Eugene in June, won his heat and clocked 10.01s to advance to the semifinals, with fellow American Justin Gatlin joining him.
Yohan Blake was beaten in the second race as Abdul Hakim Sani Brown registered a personal best of 10.05s.