Jamaican sprint star Usain Bolt won his 100metres heat after what he called a "sluggish" start on Saturday, but looked pained after the race as he prepared to fend off the challenge of American Jason Gatlin for the Olympic crown.
Bolt, who turns 30 on the final day of the Games, is aiming high in what he has said will be his final Olympics, bidding to continue his streak of gold in the men's 100m, 200m and 4x100m relay, the unprecedented "triple-triple."
The superstar qualified in a leisurely 10.07 seconds, picking up the pace after lumbering from the blocks and looking stiff in his warm-up.
"It wasn't the best start, I feel kind of sluggish. I think it's the fact that it's the morning. I'm not usually running this early in the morning," Bolt told reporters after the race that began at 12:42 pm local time in warm sunshine.
"Hopefully tomorrow I'll feel much better," he added of Sunday's action when the semis and final take place late in the evening.
Gatlin, 34, turned in the fastest time of the morning, 10.01 as the 2004 champion seeks to become the oldest man to win an medal in the 100m and the first to do so after serving two doping bans.
"I felt good. I think I'm going to have to run a bit faster (than my season's best of 9.80 seconds) to win this medal," Gatlin said.
Ben Youssef Metie, 29, of the Ivory Coast, notched the morning's second-faster time of 10.03, edging U.S. hopeful Trayvon Bromell's 10.13, which was still fast enough to quality. Canada's de Grasse, a 21-year-old who only took up running seriously three years ago after an early focus on basketball, ran a 10.04.