Sanya Richards fears Chrissy Ohuruogu will thwart her World Championships gold medal hopes in Berlin.

This was despite her current dominance of the women's 400 metres.

Richards defeated Britain's Olympic champion decisively at the Oslo Golden League meeting a fortnight ago and the pair should have had another rematch at tonight's event in Paris.

However, Ohuruogu has accepted medical advice to rest a tight hamstring and will not run at the Stade de France where Richards will be aiming to place her name in the record books.

Should she run under 50 seconds the 24-year-old American will surpass the achievement of legendary east German 400m runner Marita Koch, who achieved 36 such times during her magical career between 1977 and 1986.

Richards is as excited at the prospect of achieving the feat as she is eager to keep a clean sheet for the fourth occasion and continue her quest to win the US dollars one million dollars jackpot for remaining unbeaten in the six GL events.

However, the 2005 world silver medallist, who failed to make Team USA for the championships two years later and was beaten into third place in last summer's Olympic Games, insists winning in Berlin next month is her top priority.

"For now I just want to run whatever it takes to win my first world title," Richards said.

"I feel really good and my training has been going really well. This year it's all coming together.

"My coach and I are on the same page. The races have become easier.

"Hopefully I can carry it into the World Championships."

Richards, beaten by Ohuruogu and Shericka Williams at the Beijing Olympics, admitted: "I do really well at Golden League meets - it's always perfect but not at championships.

"Christine Ohuruogu will go out and defend her title as favourite. I'm going out as underdog, although that's not strictly true.

"I'm just using that mentally so as not to put pressure on myself."

The Berlin favourite, insisted: "Ohuruogu and Williams are proving they run well at championships so they will be at their best in Berlin. So I give them credit for this."

Olympic triple jump silver medallist Phillips Idowu faces a strong field of Cubans who are ahead of him in the world rankings but is undeterred by the challenge.

"I'm in great shape and the field is quite tasty but hopefully I will cope with them," said Idowu.

"Look at my results, I've never had a season like this."

Simeon Williamson, who faces world 100m record holder Usain Bolt, has played down his chances of breaking the 10-second barrier for the first time, even after clocking 10.05secs into strong headwind when winning last weekend's UK title.

Williamson accepts his performance equates to a time of 9.90sec in normal conditions, but said of the straight final: "I'm someone who performs better when there are rounds. But I'll be looking for a good run."

Tim Benjamin, who missed the trials, will be aiming to impress the selectors with a fast time in his race as will Marilyn Okoro who was third over 800m in Birmingham.

Michael Rimmer and Andy Turner, whose victories there clinched their World Championship places, compete over 800m and the 110m hurdles.