The 22-year-old Pole, who is the defending champion at Flushing Meadows, defeated Slovenia's Kaja Juvan 6-0, 6-1 in the third round on Friday.
The Polish tennis star said after the match: "I didn’t like the fact that I was winning against my best friend, but I knew that I had to be really focused and not let myself think about that because it was like playing against a sister."
Świątek added: "We’ve known each other for so long. I don’t have many friends, but she’s my best friend. That was the toughest part, for sure."
Świątek will now face Latvian Jelena Ostapenko for a place in the quarterfinals of the hard-court tournament in New York.
Meanwhile, Poland's world No. 24 Magda Linette and 77th-ranked Magdalena Fręch both crashed out of the US Open women's singles in the second round.
On the men's side, Poland's 17th-seeded Hubert Hurkacz was beaten by Britain's Jack Draper 2-6, 4-6, 5-7 in the second round on Thursday.
The 2023 US Open runs until September 10.
(gs)
Source: Eurosport, PAP, usopen.org
