Michael Phelps was targetting a 20th Olympic medal on Thursday while the Games' badminton match-fixing scandal prompted disgraced Chinese star Yu Yang to announce her shock retirement.

Phelps faces a dogfight with US team-mate Ryan Lochte in the 200m individual medley.

Lochte will start favourite to win his second individual gold medal of the Games but Phelps wants to win at least one individual title in London.

The 27-year-old finished second to Lochte when they met in the semi-finals on Wednesday, and Lochte has the mental advantage of his crushing defeat of Phelps in Sunday's 400m individual medley.

But the pressure is now off Phelps since he overtook the 18-medal haul of Soviet gymnast Larisa Latynina with two medals on Tuesday night, including the relay gold which clinched the Olympic milestone.

After Britain finally won its first two golds of the Games on Tuesday, its highly successful track cycling squad will hit the futuristic Velodrome where a packed 6,000 crowd will roar them on.

The men's and women's team sprint finals will be contested on the opening day of the six-day competition and Britain are among the favourites.

Meanwhile, Yu Yang, one of the eight players disqualified from the Olympics for throwing a badminton match announced she is quitting the sport, saying her dreams had been "heartlessly shattered".

Yu, who won a gold medal in the 2008 Beijing Olympics, and her partner Wang Xiaoli were disqualified by the Badminton World Federation on Wednesday for "not using one's best efforts to win a match".

"This is my last time competing. Goodbye Badminton World Federation, goodbye my beloved badminton," Yu Yang said on her Weibo, or microblog.

"You have heartlessly shattered our dreams."

On the eve of the start of action in the athletics, Jamaican sprint contender Yohan Blake is fielding the inevitable questions about his rivalry with training partner Usain Bolt.

Blake has had the edge over double Olympic champion Bolt this year, beating him in the 100m and 200m in the Jamaican trials, but he insists he will focus on his race and not his illustrious friend.

"I've got no message for Usain Bolt. He's a good guy. I'm not focusing on Usain," Blake said.

"It's all about going out there, focusing and executing the race."

In the gymnastics arena, Gabby Douglas of the United States looks a contender for the women's all-around title after Japan's Kohei Uchimura won the men's gold on Wednesday.

Meanwhile, the Dream Team will expect to book their place in the next round of the basketball tournament when they face Nigeria.

Only Spain and their LA Lakers centre Pau Gasol look capable of stopping the collection of NBA superstars.

In all, 18 medals will be decided on Thursday.