BEIJING, August 17, 2008 (AFP) - American world number one Sanya Richards and Britain's world champion Christine Ohuruogu successfully sailed through their 400 metres semi-finals here on Sunday to set up a potentially explosive head to head in Tuesday's final.

Richards, who missed out on competing in the world championships because she was struggling with the rare Behcet's disease, recorded the fastest time of the semi-finals of 49.90 seconds.

Ohuruogu, who Richards said was fortunate to be competing here after serving a one year ban for missing three doping tests and under BOA rules should not be allowed to represent Britain in an Olympics, timed 50.14sec to win her semi-final.

However, there were three major casualties as Ohuruogu's compatriot Nicola Sanders, silver medallist in the world championships, and Jamaica's Novelene Williams, bronze medallist at the world's, failed to reach the final.

American Deedee Trotter had a woeful semi-final, the 25-year-old two-time world and 2004 Olympic finalist being outpaced from the start and came in well down the field behind Russia's Yuliya Gushchina.