BEIJING (AFP) - Jamaican Usain Bolt, fuelled by high-octane chicken nuggets and yam, bids to be first man in 24 years to win the Olympic sprint double on Wednesday, as the Games were hit by their first high-profile drugs failure.

Ukraine's Olympic heptathlon silver medallist Liudmyla Blonska has failed her A sample, the International Olympic Committee (IOC) said.

An IOC source told AFP they were awaiting the results of the B sample before passing their recommendations to the executive board which will meet on Thursday.

"There has been an adverse finding in her A sample against her (Blonska)," said the source.

If the B sample is also positive it would be the fifth case of the Games.

The IOC had claimed earlier in the day it was winning the battle against drug cheats after conducting 4,133 tests so far in Beijing.

Of the four previous positive cases detected only one involved a podium finisher - North Korean shooter Kim Jong-Su, a minor medallist.

Bolt, meanwhile, heads into the 200m final predicting he will repeat the double last achieved by Carl Lewis in 1984.

The Jamaican sensation ran a world-record 100m final after a feed of chicken nuggets, and attributes his phenomenal speed to growing up on a diet of his mother's yam mash.

The 21-year-old Bolt owns the three fastest 200m times this season, ran an effortless semi-final on Tuesday and joked afterwards he was only jogging.

"I was just trying to make sure I was in a good position, keeping an eye on the board and seeing where everyone else was," he said, but indicating Michael Johnson's world record of 19.32 seconds may be out of reach.

"I'm not really worried about the world record," Bolt said. "I'm just coming here to win. That's the aim for me."

Defending champion Shawn Crawford and 100m bronze medallist Walter Dix, both members of a US track team that has failed to excel in Beijing, figure as Bolt's main rivals.

On day 13, one of the quietest days at the Olympics, there are only 11 finals in six sports - athletics, sailing, open-water swimming, synchronised swimming, taekwondo and wrestling.

China continues to hold a commanding lead on the medal's table with 44 compared to 26 for the the United States and 16 for Great Britain with 93 finals remaining before the Games close on Sunday.

Yin Jian lifted China further ahead winning the women's windsurfing, but the Games hosts had no luck in a preliminary round baseball match when crushed 17-1 by Cuba with the match called off in the seventh inning under the sport's mercy rule.

Sheena Tosta and Tiffany Ross-Williams front as the best US prospects on the track in the women's 400m hurdles final,

But none of the leading sports nations are likely to stop Jamaica getting its second Bolt gold and third of the Games in the day's feature event.

Russian open water superstar Larisa Ilchenko entered the record books as the first women's 10km swimming marathon champion when the sport made its debut on Wednesday.

Ilchenko, unrivalled in open water swimming for the past four years, overtook Great Britain's Keri-Anne Payne in a sprint finish to win the gruelling event in just under two hours in the water.

South Africa's Natalie du Toit, the first amputee to qualify for the Olympic Games, finished 16th in the 25-strong field and pledged to be back in 2012 to do better.

"I don't even think about not having a leg and if I want to keep competing I will have to continue to qualify with the able-bodied. For me it's not about the disability at all," said du Toit who lost her left leg in a motorcycle accident seven years ago.

Another sport making its Olympic debut, BMX racing, saw American professional Mike Day coming out tops from the men's qualifying rounds.