Gauff, a 22-year-old American, will play Wednesday for a berth in Thursday's final against the winner of a match between four-time Grand Slam champion Naomi Osaka of Japan and world number two Elena Rybakina of Kazakhstan.
Swiss star Bencic, ranked 14th in the world at age 29, pulled out with the US Open set to start at the end of the month. Bencic has never reached a Grand Slam final.
Gauff, 6-2 all-time in WTA matches against Bencic, had beaten her this year in the fourth round at Wimbledon and the Miami quarter-finals.