Lia Thomas, representing the University of Pennsylvania, won Thursday morning's heats with a time of 4:33:82, edging out Erica Sullivan (Texas, 4:36.79) and Emma Weyant (Virginia, 4:37.25).
She will be the first transgender athlete to win a Division 1 title if she wins the final at the McAuley Aquatic Center.
She also had the fastest time in the 200-yard freestyle in 1:41.93 and was ranked 10th in the 100-yard freestyle in 47.63.
Thomas, who had competed as a man at Penn State, divided opinion.
Some feel she has an unfair physiological advantage, while others believe she should be allowed to compete freely as a woman.
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