The Swiss Federal Supreme Court has reversed its ruling to temporarily suspend the International Association of Athletics Federation's rule on testosterone limits for Olympic and world champion Caster Semenya. Semenya appealed the decision by the Court of Arbitration for Sport on May 1 and was able to compete once in the women's 800 meters in June.

The decision will prevent Semenya from defending her world championship gold medal at this year's IAAF World Championships in Doha, Qatar.

"I am very disappointed to be kept from defending my hard-earned title, but this will not deter me from continuing my fight for the human rights of all the female athletes concerned," Semenya said in a statement.

CAS ruled in favor of the IAAF in the implementation of new regulations to limit testosterone levels for athletes with a difference in sex development. The decision means that Semenya and other athletes will have to take medication to reduce their naturally-high testosterone levels if they want to compete against other women at the international level. CAS believed the rule was "necessary, reasonable and proportionate means of achieving the IAAF's aim of preserving the integrity of female athletics." Under the new CAS rule, athletes with differences in sexual development will have to keep their testosterone levels under 5 nanomoles per liter. In the IAAF's case, they noted that elite female athletes tend to have natural testosterone levels of approximately .12 to 1.79 nanomoles per liter. If an athlete with naturally higher levels of testosterone opts not to take medication, they will have to race against men or compete in a division against intersex athletes—if that is presented as an option. The rule will impact athletes in events from 400 meters to the mile.

The appeal by the Swiss Supreme Court only applied to Semenya. She raced at the Prefontaine Classic in late June and won the women's 800 meters in 1:55.70 – more than two seconds ahead of United States' Ajee Wilson in second place. Semenya's 1:54.98 from the Doha Diamond League in May is the fastest time run by a woman in 2019. Wilson's 1:57.72 from her victory at this weekend's U.S. Outdoor Track and Field Championships is the second-fastest time of the year and could make her the favorite for gold. Wilson won bronze at the 2017 IAAF World Championships and looks to become the first American woman to win the event.

The South African star has gone undefeated in 31 800-meter races since 2015. The last time Semenya was not the world champion was 2013 when she did not compete and Kenya's Eunice Jepkoech Sum won gold.

Olympic silver medalist Francine Niyonsaba of Burundi and bronze medalist Margaret Wambui of Kenya have not competed since the CAS ruling because they are also affected.