Searing heat, a lightning fast track and a stellar start by Sweden's Carolina Kluft heralded the opening day of the 11th world championships on Saturday.

Tyson Gay and Asafa Powell raced through stifling Japanese temperatures to ease towards their much-anticipated 100m showdown.

Heptathlete Kluft recorded personal bests in the hurdles and high jump and Luke Kibet became the first Kenyan man to win the world marathon title in 20 years.

Gay, the fastest sprinter this year, and world record holder Powell cantered safely through to the second round of qualifying, both easing up at the end.

"I tried to run 10.2 or 10.3 but I couldn't run that slow," Gay quipped. "It's the fastest track I've ever run on," the American added after clocking 10.19.

Powell ran 10.34. There was a shock for Portugal's Francis Obikwelu, though, as the Athens Olympic silver medallist was disqualified for a false start.

Britain's Marlon Devonish was the quickest man in the first round, qualifying in 10.13.

While the track is proving fast, searing heat and sapping humidity produced the slowest men's marathon winner in 11 world championships.

ECSTATIC KLUFT

Kibet took gold in 2 hours 15 minutes 59 seconds. The fastest man in the world this year, Qatar's Hassan Mubarak Shami, held on for silver with Switzerland's [Images] Victor Roethlin taking bronze.

"I'm happy and I am proud to win the gold medal for my country," Kibet told reporters. "It's been a long time for Kenya .without the gold medal in marathon," he said after emulating compatriot Douglas Wakiihuri's 1987 feat.

The thermometer had already tipped 29 degrees Celsius at the 7am start and had climbed to 33 degrees by the finish.

Sweden's Kluft enjoyed her morning in the sunshine, taking her customary position at the top of the heptathlon leaderboard after two events.

Following a personal best of 1.95 metres in the high jump she skipped off the mat and wheeled around the field skipping and circling her arms.

Her 13.15 in the 100m hurdles was also a best and after two events she held a 51-point lead over Britain's Jessica Ennis.

Looking to extend her winning streak to 22 multi-event competitions, Kluft heads into the shot put and 200m on Saturday evening.

Two golds will be up for grabs during the session, the men's shot put and the women's 10,000 metres.

Tirunesh Dibaba, who made history two years ago in Helsinki by completing the world 5,000m and 10,000m double, should be the woman to beat.