Lia Thomas was 'dominated' by fellow transgender swimmer Iszac Henig in an Ivy League swim meet on Saturday (8 January).
The University of Pennsylvania swimmer was defeated twice by Henig, of Yale University, who is transitioning from female to male.
Henig is allowed to compete in women's events as he has delayed hormone treatment in order to finish his swimming career on the Yale women's team.
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Record-breaking Thomas lost out to Henig in both the 100-yard freestyle, finishing fifth, and then again in the 400-yard freestyle relay, as per the Daily Mail.
The defeat will have come as a shock to some as it was only last month that Thomas beat two US records at the Zippy Invitational Event in Akron, Ohio.
However, this time round, it was Henig who came out on top, finishing the women's 100-yard freestyle with a time of 49.57 seconds.
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Thomas finished fifth with a time of 52.84 seconds.
In the women's 400-yard freestyle relay, Henig picked up the win yet again, after finishing in 50.45 seconds, taking Yale to victory.
Thomas finished her leg of the race in 51.94 seconds.
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Henig, the 20-year-old from Palo Alto, California wrote in a New York Times column in June 2021: "As a student-athlete, coming out as a trans guy put me in a weird position.
"I could start hormones to align more with myself, or wait, transition socially and keep competing on a women's swim team. I decided on the latter."
He added: "I value my contributions to the team and recognize that my boyhood doesn't hinge on whether there's more or less testosterone running through my veins.
"At least, that's what I'll try to remember when I put on the women's swimsuit for the competition and am reminded of a self I no longer feel attached to."
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In December, Thomas, who used to compete as a man, sparked controversy after braking two records.
Speaking about the backlash she received following her victories at the tournament, Thomas said she has worked incredibly hard to be where she is.
She told SwimSwam: "I've experienced a lot of muscle loss and strength loss. [Swimming has] been a huge part of my life for as long as I can remember.
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"Pre-transition there was a lot of of uncertainty about my future in swimming and whether or not I'd be able to keep swimming at all and so I'm just thrilled to be able to continue to swim.
"I love to compete and I just love to see how fast I can go. It's sort of an ongoing evolution of what I think I can go based on how my training sort of progresses and evolves."
Featured Image Credit: YouTube/SwimSwamTopics: Sport