On Wednesday night, Fernandez defeated world number five Elina Svitolina, Ukraine, in the quarterfinals 6-3, 3-6, 7-6.
She has previously knocked out world number three Naomi Osaka and three-time grand slam winner Angelique Kerber from the women's singles tournament at the US Open.
- Honestly, I have no idea what I feel right now, Fernandez said immediately after the quarterfinal win.
In the semis, Fernandez meets world runner-up Aryna Sabalenka, Belarus, who practically overtook Czech Barbora Krejcikova in the quarterfinals, 6-1, 6-4.
It will be the semi-final premiere in the US Open for both of them.
Fernandez, who turned 19 earlier this week, is one of several teenagers who have shown top form in this year's USA Open.
The 18-year-old Spaniard Carlos Alcaraz had also reached the quarterfinals of the men's singles tournament where he met the Canadian Felix Auger-Aliassime.
But he interrupted the match in the second set due to a leg injury, Auger-Aliassime then led 6-3, 3-1.
21-year-old Auger-Aliassime meets Russian Daniil Medvedev in his first semifinal in the grand slam context.
Another 18-year-old who has a chance at the semifinals is the British Emma Raducanu, who meets the Swiss Belinda Bencic in her quarterfinals.
- I think we are all super eager to make a difference in the tennis world, says Leylah Fernandez about the teenagers' success.
- I have known some of them from the junior tours.
We have always talked and joked that we should stand on the big stages together.