The third-seeds defeated Caty McNally of the United States and Lorenzo Musetti of Italy 4-1, 4-2 in the quarterfinals on Tuesday.
Earlier in the day, they beat Americans Madison Keys and Frances Tiafoe by the same score in the opening round.
Świątek and Ruud will face top seeds Jessica Pegula of the United States and Jack Draper of Britain for a place in the final on Wednesday.
This year, the mixed doubles competition is being held in New York during US Open qualifying week rather than alongside the singles competition.
Organizers said the change was aimed at raising the event's profile and attracting top players.
Matches use a faster format. Each set is played to four games instead of six, and at 40–40 the next point decides the game. If the score reaches 4–4, a tiebreak is played. If the match goes to a deciding set, it is settled by a first-to-10 super tiebreak.
Only the final will follow the traditional six-game set format, Polish state news agency PAP reported.
(gs)
Source: IAR, PAP