The 20-year-old Czech backed up her fourth-round win over world number one Aryna Sabalenka with a determined victory over Keys, a former Australian Open champion who won the title in Cincinnati in 2019.
Bejlek got off to a slow start, trailing 0-3 on the way to dropping the first set, but rode a wave of momentum shifts into the final four, where she'll face either fourth-seeded Coco Gauff or Marta Kostyuk.
The youngster broke first in the third set only for Keys to battle back and force the tiebreaker, in which Bejlek rallied from 4-2 down on the way to a victory capped by a stinging winner down the line on match point.
Bejlek, who won the title in Abu Dhabi in February as a wild card, continued a spectacular Cincinnati debut.
After a solid first set, the 24th-ranked Keys opened the second on sweltering Stadium Court with a break in the third game, but Bejlek, ranked 35th, broke back in the sixth and held for a 4-3 lead.
Keys answered with a love game, but Bejlek turned the tables two games later, taking the contest into the third set as Keys hit the net with a backhand on the Czech's second set point.