RIO DE JANEIRO • Usain Bolt vowed to go for a world record in the Olympic 200m final this morning (Singapore time), as he looked unstoppable in Wednesday's semi-finals, clocking a season's best of 19.78sec.

He even had time to ease up 40 metres from the line and still finish ahead of Canada's Andre De Grasse, who tried to beat the surprised Jamaican but was 0.02sec slower.

The duo exchanged smiles after the race, and Bolt said: "He was supposed to slow down. I said, 'What are you doing? It's a semi-final.'

"But I think he wanted to push me. I was a bit lazy but I got round."

Nevertheless, he believes his 19.19sec world mark is in jeopardy.

"I definitely think I can try for the world record. I definitely feel that," he said. "I need to run efficiently and get into the straight and run the perfect race."

Several factors would have to fall into place for that to happen, he said, including a draw of the sixth or seventh lane, to take advantage of the more gradual curve on the outside of the track.

There was a shock as Bolt's rival Justin Gatlin - the fastest man over the distance this year - failed to make the final.

The American 100m silver medallist clocked 20.13, a mere 0.03sec slower than the eighth-best qualifier, Martina Churandy of the Netherlands.

Gatlin blamed an ankle injury suffered in November, saying: "It's been flaring up this season. Tonight it got really stiff on me in the warm-up area. Having that lane three coming off the turn, I felt pain in the last 20 metres. I tried to hold on and do the best I could."

REUTERS, AGENCE FRANCE-PRESSE