Michael Phelps [Images] scorched to his sixth gold and his sixth world record of the Beijing [Images] Olympics [Images] on Friday, one of three wins in the Water Cube for the United States.

North Korean Kim Jong-su, who won silver and bronze in shooting, became the first medalist to test positive for drugs, and the International Olympic Committee said his medals had been rescinded.

• Michael Phelps, the billion dollar man?

Phelps now has 12 career Olympic golds, three more than anyone else, and is now chasing Mark Spitz's [Images] 1972 record of seven golds at a single Games -- all of which also came in world best times.

The 23-year-old Phelps showed little reaction apart from a quick shake of his fist after victory in the 200 meters individual medley on Friday, but burst into a broad smile on the podium later.

• Phelps, the Phenom

"I just wanted to step on it in the first 50 a little bit and try and get out to an early lead," said Phelps. "I knew in the first half if I got a big enough lead I thought I could hang on and that's all I wanted to do."

He races in the 100 butterfly final just after 10 a.m. (0730 IST) on Saturday morning, and if all goes well could be in the running to break Spitz's record in the 4x100 medley relay final, to be held just before 11 a.m. in the Water Cube on Sunday.

In the next-door Bird's Nest stadium, athletics action finally began on Friday under blue skies, with Usain Bolt [Images], Asafa Powell [Images] and Tyson Gay [Images] coasting to victory in their heats of the men's 100 meters.

The United States team hope the track and field events will give them a chance to catch up with China in the battle for gold medals, to add to a growing haul of golds in the pool.

On Friday America's Ryan Lochte set another world best time as he won the men's 200 backstroke, from compatriot Aaron Peirsol.

Team mate Rebecca Soni also won the women's 200 meters breaststroke in a world record time, coming from behind to overtake the 100 meters winner Leisel Jones from Australia [Images].

"I had plenty left in me," said Soni, who underwent minor heart surgery two years ago. "It's been a long road to get here, and I really can't believe that just happened."

Jones, favorite to win her second gold, said she was "digging deep" at the end but just could not breathe.

It was a double disappointment for Australia's women, dominant in the pool until now, when world champion Libby Trickett was also overhauled right at the finish by Germany's [Images] Britta Steffan in the 100 freestyle.

Trickett, who won gold in the 200 meters event, immediately hugged and congratulated Steffan in the next lane.

The quest for dominance in the medals table is proving a fascinating contest between the United States and China, which came second in Athens in 2004. So far China leads 22 golds to 13.

The Communist Party is desperate to underline the country's growing superpower status by overtaking the United States, and its athletes has been playing to strengths in events like table tennis, diving, gymnastics and weightlifting.

ETHIOPIA CARRIES AFRICAN HOPES

But the gap may narrow when action gets under way in the Bird's Nest, and Friday's shot put contest gives the U.S. squad a great chance for a morale-boosting counterattack.

It has been 48 years since one nation swept medals in that event, but US trio Adam Nelson, Reese Hoffa and Christian Cantwell have the skill to match their 1960 compatriots.

Medals will also be awarded on Friday in the women's 10,000 meters, where Ethiopia's runners dominate and could bring some cheer to Africa which has had a disappointing Games so far.

Tirunesh Dibaba, who has two world championship titles, is favorite. She expects to run the 5,000m too and hopes to become the first woman to win the Olympic distance double.

"My expectation is that I will run both," she said. "It's being said that it's a little hot here, so the final decision will be made after the 10,000."

Away from the Bird's Nest and Water Cube venues, the fixture list again threw up some politically intriguing contests.

In baseball, Cuba face arch-enemies the United States, while China take on Taiwan who they regard as a breakaway province.

The Games lost a dash of glamour when three of the biggest names in tennis, Roger Federer [Images] and U.S. sisters Venus and Serena Williams [Images], crashed out at the quarter-final stage on Thursday.

Security has been gradually ratcheted up at the Games, since the father-in-law of the American volleyball coach was stabbed and killed in Beijing on the first full day of the Games.

X-ray and scanning machines have now been deployed at the most heavily visited section of the Great Wall just outside Beijing, an official said on Friday.

Sporadic protests have continued in Beijing despite the clampdown. Foreign activists unfurled a banner proclaiming "Free Tibet [Images]" over an Olympics poster at the newly built headquarters of China's state television broadcaster on Friday.