BERLIN: Olympic champion Usain Bolt and reigning world champion Tyson Gay cruised into the semi-finals of the men's 100m on the opening day of the World Athletics Championships here on Saturday.

In sultry conditions at the Olympic Stadium, it was Bolt's team-mate and former world record holder Asafa Powell, however, who set the pace.

The bronze medallist from the 2007 Osaka worlds looked very comfortable when racing home to win his quarter-final in 9.95sec and refused to ease up as he had done in the opening heats when he was beaten into third.

"I'm fine. I just wanted to get through the day," Powell said. "The first round in the morning I was just running too easy and I underestimated the other guys." Bolt was beaten into second in his heat by training partner Daniel Bailey of Antigua, with the two sprinters easing up and even laughing together as they crossed the line in just over 10sec.

"I'm tired. It's been an exhausting day," said Bolt.

In the US camp, defending triple world sprint champion Gay cruised through in the second fastest time of 9.98sec along with his trio of team-mates Darvis Patton, Michael Rodgers and Monzavous Edwards.

But the visibly limping Gay again complained of his nagging injury. "My groin is sore but, all in all, it went pretty well," he said.

In what is building up to be a USA v Jamaica sprint-off, Jamaican Michael Frater also qualified for Sunday's semi-finals, slated for 0955 GMT, with the final scheduled for 1935 GMT.

Briton Dwain Chambers, the world indoor 60m silver medallist who is competing here after having served a two-year doping ban, won his heat in a season's best of 10.05sec ahead of Richard Thompson of Trinidad.

"I'm just glad to get through," Chambers said. "This is a real confidence boost for me in getting through to the semis.

"I can't worry about what's happened in the past (the doping) when I get to a championships. I have just got to try and concentrate and not get caught up with anything else. My main target is a medal." There were tears for promising 19-year-old French hope Christian Lemaitre, however, after he was disqualified along with Colombian Daniel Grueso for a second false start.

The one shock result in the 12 opening heats earlier in the day came as the silver medallist from Osaka, Derrick Atkins of the Bahamas, failed to qualify after clocking a disappointing 10.44sec.

In the morning session, Russian Olympic champion Valeriy Borchin claimed the first gold of the championships when he won the men's 20 kilometres walk title ahead of China's Wang Hao and Mexican Eder Sanchez.

After the first three events of the heptathlon, Britain's Jessica Ennis remained in pole position on 3,070 points having recorded field leads of 12.93sec in the 100m hurdles and 1.92m in the high jump, and a personal best of 14.14m in the shot put.

An impressive season's best of 15.82m saw Olympic champion Nataliya Dobrynska move from sixth spot to second on 2,932 points.

The heptathletes will later compete in the 200m. The second day of competition on Sunday involves long jump, javelin and the 800m.

In the women's 400m, American Sanya Richards struck the first blow in her bitter rivalry with Britain's Olympic and world champion Christine Ohuruogu, winning her heat in a conservative 51.06sec.