By Scott Malone and Gene Cherry

RIO DE JANEIRO (Reuters) - Jamaican Usain Bolt won his 100 meters 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 Justin 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".

He 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 which began at 12:42 p.m. 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.

He batted back questions about his leg, saying he was "good".

Gatlin, 34, produced the fastest time of the morning, 10.01 as the 2004 champion seeks to become the oldest man to win a medal in the 100m and the first to do so after serving two doping bans.

"I am just staying focused, I went out there and executed my race in the first round and cruised to finish line," Gatlin said.

Asked if the race felt more special, given that it is likely to be his last Olympics, Gatlin replied: "Every Olympics is special but as you get older you understand the importance of running at Olympics."

Ben Youssef Metie, 29, of the Ivory Coast, notched the morning's second-fastest time of 10.03, edging U.S. hopeful Trayvon Bromell's 10.13, which was still fast enough to qualify.

Canada's Andre de Grasse, a 21-year-old who only took up running seriously three years ago after an early focus on basketball, ran 10.04.

(This version of the story corrects Justin Gatlin's first name in first paragraph)

(Additional reporting by Kayon Raynor, editing by Mitch Phillips and Ed Osmond)