AFP/ Beijing

Usain Bolt icily dismissed the threat of American Justin Gatlin after the fierce rivals cruised into the semi-finals of the 200 metres at the world championships on Tuesday.

The Jamaican superstar, who pipped two-time doping offender Gatlin to win gold in Sunday's final in Beijing, eased up with some 50 metres to go after coming off the bend well ahead of the field to win his heat in 20.28 seconds.

Gatlin, game-face on after failing in his bid to dethrone Bolt as the king of men's sprinting, won his heat in 20.19.

But Bolt, whose 100m victory gave athletics a boost after allegations of widespread doping plunged the sport into crisis, shrugged when asked if was confident of beating Gatlin once more to retain his double world sprint title.

"I'm always confident, man," sniffed the six-times Olympic champion and world record-holder. "I knew the 200 was going to be harder because I'm not in the best of fitness," added Bolt, who struggled with pelvic joint pain earlier in the season.

"Definitely I was trying to make sure I ran at least 130 metres of the race and then I kind of shut it down. Everybody knows the 200 actually means more for me than the 100 so I'm trying to get through the rounds as comfortably as possible and using as little energy as possible."

Azeri-born Turk Ramil Guliyev grabbed his moment in the spotlight to record the fastest time of the heats in a national record 20.01.

- 'Difficult race' -

"The race felt good," said Gatlin, who had been unbeaten in 28 races stretching back two years to the Moscow world championships before being floored again by his nemesis Bolt.

"The 100m final was a difficult race for me, also emotionally. I made some mistakes at the end of the race but now I'm going for the 200. I have two more days to go."

Among those who also advanced to Wednesday's semi-finals were 16-year-old Japanese schoolboy Abdul Hakim Sani Brown, who clocked 20.35 behind Gatlin to earn the American's respect.

"My race would have been slower but Sani pushed me so much," said the 33-year-old Gatlin. "This young guy from Japan is phenomenal."

Sani Brown, who shot to fame in Japan by claiming a sprint double at last month's world youth championships, was almost lost for words.

"It's a dream for me to run at the world championships with these stars," he grinned. "I'm just very grateful to be here. I was a bit ragged at the end but I think I can go faster."

Born to a Japanese mother and Ghanaian father, Sani Brown insisted he had not felt nervous about racing alongside Gatlin.

"To be honest I didn't feel like I was racing him at all," said the youngster, who was ordered by his mother to choose track over football when in elementary school. "It was just a great experience."

The 200m final takes place on Thursday at the Bird's Nest stadium.