Britain's Lynsey Sharp says the rest of the world's 800m runners will be racing for second place behind South Africa's Caster Semenya at the Olympic Games in Rio de Janeiro in August.

"It's two races, so there's nothing I can do, there's nothing anyone else can do, it's out of our control," Sharp told British media after the 25-year-old finished fourth in a Diamond League race in Birmingham on Sunday.

"I know I can run faster than what I've already run but I can't control what other people do," the 2012 European champion said.

Semenya was subject to gender testing after winning gold at the World Championships 2009 in Berlin and was then required to take hormones to lower her abnormally high testosterone levels.

However, the Court of Arbitration for Sport overturned that ruling and the 25-year-old has since become a strong favourite for gold in Rio after setting the fastest time of the year in Rabat, Morocco, with 1minute, 56.64 seconds.

Semenya is also planning to run the 400m in Brazil, She also won the South African Championships in a competitive 50.78 seconds in April, making her a strong medal contender at that distance as well.

Meanwhile, the SA contingent struggled to hit top form at the Birmingham meeting on Sunday.

Javelin thrower Sunette Viljoen settled for seventh place with a best heave of 58.54m. Sprinter Carina Horn also battled, clocking 11.57 seconds for a fourth place in her 100m heat.