Tauson dominated the opening set and took full advantage of Keys giving up a break on a double fault to open the second. The Dane won 12 of 19 points on her second serves and kept Keys at bay most of the night.
Keys joined a scrap heap of Montreal seeds, with the top five seeds and seven of the top 10 seeds ousted before the quarter-finals.
Tauson, 22, seeks her fourth career WTA title after the 2021 Lyon and Luxenbourg Opens and this year’s Auckland Open.
The Dane, who also eliminated Polish second seed Iga Swiatek in the fourth round, is one win from her second WTA 1000 final, having lost the first in February at Dubai to Mirra Andreeva.
She dominated Keys in the first set, breaking in the fourth and sixth games, and Keys double faulted away another break to begin the second set.
Tauson held from there to win, never facing a break point in the second set before holding at love in the last game to end matters after 70 minutes.
Elena Rybakina, the 2022 Wimbledon champion, and 18-year-old Canadian sensation Victoria Mboko will meet in Wednesday’s other semi-final on the Montreal hardcourts in the US Open tuneup.
Rybakina won their only prior meeting 6-3, 7-5 last month in the second round at Washington. (AFP)