Semenya crowned a spectacular season by triumphing in 1min 55.45sec while Britain’s Jenny Meadows produced a lifetime best of 1min 57.93sec to take the bronze – Britain’s third medal of the championship. Fellow Briton Marilyn Okoro was eighth.
But instead of being able to celebrate her victory, Semenya found herself facing uncomfortable questions about whether she should really have been lining up in today’s first-round heats of the men’s 800m.
Ever since her arrival in Berlin, she has been the subject of whispers and innuendos about her masculine body shape and facial features.
Earlier in the day the International Association of Athletics Federations admitted that it was so concerned about the rumours and her huge improvement this season that it had asked Athletics South Africa (ASA) to carry out a "gender verification" test on the athlete.
IAAF spokesman Nick Davies said: "Following her breakthrough performance three weeks ago at the African Junior Championships, where she made a tremendous improvement for someone who is 18, the rumours, the gossip, the challenges if you like, started to build up.
"At that point the IAAF contacted the South Africans and asked them if they could they demonstrate gender verification documentation and, if not, that they should start to provide something."
Davies said doctors in South Africa had already begun testing Semenya but because of the complexity of the process, it would be some time before the results were available. Until then, she was clear to run.
"We need to make sure that the rules are followed and clearly women should compete in women’s competition," he said.
But the questions about Semenya’s gender were dismissed by ASA spokeswoman Ethel Manyaka, who said the federation would not have sent her to Berlin if it was not certain she was a woman.
Semenya’s coach, Michael Seme, also rejected the rumours in an interview before the championships.
"We understand that people will ask questions because she looks like a man," he said, adding that when he and Semenya stopped at a petrol station in Cape Town recently and the teenager tried to enter the ladies’ toilets, she was barred by the petrol attendants because they were convinced she was male.
"Caster just laughed and asked if they would like her to take off her pants to show them she was a woman," he said.
But resolving whether a person is male or female is not a simple matter of physical examination – the method originally used by the International Olympic Committee when mandatory gender testing was introduced in 1968 before being phased out on ethical and scientific grounds 10 years ago.
There are numerous "intersex" conditions that do not affect external genital appearance.
If Semenya is found to have an abnormality, the issue will be whether it offers her an unfair advantage. If so, hormone treatment or surgery may be necessary.
There is also a precedent for stripping an athlete of a medal, as was the case with Indian runner Santhi Soundarajan, who lost her 800m silver medal after failing a gender test at the 2006 Asian Games in Doha.
The fact that Soundarajan later attempted suicide underlines the sensitivities involved, and Davies said Semenya’s test results would remain confidential.
He said: "We have to very sensitive to this because there is a health issue and she is a human being who was born as a woman and who has grown up all her life as a woman but who is now in a position where this is being questioned."