Against Mirnyi and Oswald, who were alternates and got into the main draw when Lleyton Hewitt and John Peers pulled out, Sitak and Qureshi saved six of eight break points against their serve and played a terrific match tie break.
Some two and a half hours later they were back on court for their quarter-final and dominated from the outset. They broke early in the first set and held comfortably.
Sitak and Qureshi went up a double break at 3-0 in the second set and looked to be in total control before Sitak dropped his serve with a double fault and Demoliner and Gonzalez sniffed a comeback.
But they were unable to break again and Sitak and Qureshi converted their fourth match point on their opponents serve to close out the match and secure a place in the semis.
Fellow Kiwi Marcus Daniell and his Dutch partner Wesley Koolhof have lost their quarter-final 6-4 6-3 to second seeds Dutchman Jean-Julien Rojer and Romania's Horia Tecau.
After losing a tight opening set Daniell and Koolhof broke serve to lead 3-2 in the second set before losing four straight games to lose the match. They will now head to New York to prepare for the US Open which starts on Tuesday morning (NZ time).
Kiwi number one Michael Venus will be the third Kiwi at Flushing Meadows, teaming up with regular partner Raven Klaasen.