Bolt looks to teach a lesson in 200 final.

  Usain Bolt crossed the finish line and wagged his finger at the brash up-and-comer who dared challenge him.

Didn't Andre de Grasse know that nobody's supposed to mess with the Jamaican's 200 meters? Especially at the Olympics.

Bolt, in search of his eighth Olympic gold, considers the 200 his best race and, in the past, has taken umbrage with anyone who suggests they might beat him.

De Grasse tried to in the semifinals, even though he and Bolt were clearly in the top two spots as they came down the stretch, and securely into Thursday night's final.

Instead of slowing, de Grasse sped up and forced Bolt, who had already slowed down, to pick up the pace. The two looked at each other and smiled as they approached the finish line. Bolt leaned across in 19.78 seconds, only .02 ahead of the Canadian.

Bolt wagged his finger at de Grasse and laughed. But he didn't really find it funny.

''That was really unnecessary,'' Bolt said. ''I don't know what he was trying to do. He's a young kid, he's great. He has a lot of talent. I'm looking forward to the competition in the final.''

De Grasse conceded he pushed too hard. On Thursday, nobody will be coasting in.

As much as winning his eighth Olympic gold medal, Bolt is aiming for a world record. He holds the current mark, at 19.19 seconds, but thinks a sub-19 time could be possible.

''Now, it's (about) executing right, running the corner efficiently, and coming in the straight and running the perfect race,'' he said.

If things go terribly wrong for him, here are some others who could be a factor:

DE GRASSE: The 21-year-old bronze medalist in the 100 meters could represent a changing of the guard in the sprint game. Big question: Did pushing Bolt for a relatively meaningless semifinal placing actually sap any strength from the champion?

LASHAWN MERRITT: It's not Bolt, but Merritt, who currently has the year's best time at 200 meters (19.74 seconds). But it's not Merritt's best race. That would be the 400, where four nights earlier, Merritt staggered across the line in third, nearly a full second off of Wayde van Niekerk's world-record pace. How much did that take out of Merritt? We shall see. He'd have to be ecstatic with any kind of medal in his ''bonus race.''

CHURANDY MARTINA: The 32-year-old native of Curacao runs for the Netherlands. His claim to fame was finishing second to Bolt at the 2008 Olympics but losing the medal after officials determined he stepped outside his lane. The man who received it, Shawn Crawford, didn't think it was a fair ruling and gave Martina back his silver, though the official results stayed the same.

JUSTIN GATLIN: Um, well, no. After capturing silver in the 100, the main challenger to Bolt over the past two years didn't qualify for the 200 final. He revealed he'd been dealing with an injured ankle. But he isn't checking out completely from the race. ''I'll be out there rubbing his shoulders, with a towel on him, like, 'All right Rocky, get yourself ready,''' Gatlin said, doing his best impression of Burgess Meredith's trainer character in the ''Rocky'' movies.

THE REST: None of the following four - Christophe Lemaitre, Alonso Edward, Adam Gemili or Ramil Guliyev - have cracked 20 seconds this year. If they get caught up in Bolt's tailwind, they might.