One of them, Christine Mboma, broke the world under-20 record twice in the span of about eight hours at the Olympic Stadium on the way to the 200 final.

She also charged past American Gabby Thomas, who earlier this year became the second-fastest woman ever in the 200, down the home stretch of their semifinal. Mboma finished second behind Elaine Thompson-Herah, the defending Olympic champion.

Mboma and teammate Beatrice Masilingi aren't allowed to run in the 400 - their favoured event - after tests ordered by track governing body World Athletics found they had high natural testosterone.

That meant they fell under the same regulations that have sidelined double Olympic champion Caster Semenya of South Africa in the 800 meters.

But the testosterone rules apply only to races between 400 and the mile, allowing Mboma and Masilingi a chance at the 200 in Tokyo.

"I'm happy with my time, breaking the junior record," Mboma said. "I broke it in the morning and broke it here again. I'm really happy. It's crazy."

Asked about her ban from the 400, Mboma responded: "I have no comment for that."

Relative unknowns on the international track circuit before this year, Mboma and Masilingi will now be part of a star-studded lineup in Tuesday's final.

It will include Thompson-Herah and another Jamaican, Shelly-Ann Fraser-Pryce, who are coming off a gold-silver finish in the 100 meters. Shaunae Miller-Uibo will be there, trying to add the 200 title to the 400 she won in Rio.

Masilingi qualified a few minutes before her teammate, making it through the first semifinal with a time of 22.40 seconds. Masilingi's time was her second personal best of the day having already posted a PB of 22.63 seconds in the opening round.

"It was good, it was good. I'm happy. I'm through," Masilingi said.

The two Namibians announced themselves this season with some blistering fast times in the 400 meters, but those performances prompted World Athletics to order testosterone tests at their training base in Italy, the Namibian Olympic committee said.

The tests revealed their natural testosterone levels were higher than a limit set by the governing body, prompting their ban. The Namibian Olympic committee said the two runners were not aware they had high natural testosterone before the tests.

Athletes affected by the rules can compete in restricted races if they lower their testosterone levels for at least six months before a competition. They have three options to do that: Taking a daily birth control pill, having hormone-blocking injections or having surgery.

The testosterone rules were introduced in 2018. Athletes with testosterone levels higher than the typical range for females, World Athletics said, are deemed have an unfair advantage over other female athletes.

One of the many criticisms of the rules is that they're inconsistent because they only apply to certain race distances.

World Athletics scientists have not been able to determine whether increased testosterone has a meaningful impact in distances outside the 400-meter to 1-mile range.

Australian Associated Press