Usain Bolt felt "kind of slow" despite winning his 100 metre heat in his first race at Rio 2016.
World record holder Bolt clocked a time of 10.07 seconds to qualify for the semi-finals, the fourth fastest overall.
The Jamaican started sluggishly before powering into first position and easing off, looking to his left to ensure he stayed ahead of closest rival Andrew Fisher.
Justin Gatlin was the fastest in the heats at 10.01secs and Bolt acknowledged he has work to do if he is to be successful in his bid for an unprecedented triple-triple as he seeks gold in the 100m, 200m and 4x100m relay.
"It wasn't the best start. I felt kind of slow. I'm not used to running this early [in the day] at any championship," said Bolt. "Hopefully tomorrow [Sunday] I'll come out and I'll feel much better, much smoother.
"I'm feeling good. I'm happy. I've got the first one out of the way so I'm happy about that. So now it's all about execution and getting it right when it comes to the finals.
"It's definitely going to be good [in the] semi-finals. There are a lot of guys running fast. So there's going to be some stacked semi-finals.
"It's good. It gets you running and gets you up to speed to go out there in the finals."