Keys had to overcome a groin injury in the second set to get the job done, but she did so in style.
In a first set in which both players had their chances, it was the younger of the two Americans who made it count early on, with Keys breaking Williams in the seventh game of the opening set to take the initiative.
Keys would then go on a fantastic run which saw her win eight of the next nine points, including breaking Williams to love, to wrap up the set 6-3.
It was Keys' power hitting that was working a charm for her, as she managed 14 winners in the set to the four of Williams.
Williams would not go quietly though, and Keys endured something of a mid-match wobble as Williams went 4-1 in the second. Keys then required some attention to a left groin injury, but returned strongly after receiving attention during a medical timeout.
Keys showed she would not be slowed down by the injury as she broke to love to bring the set back to 3-4. Williams would not let her advantage slip though and took the set 6-4.
While credit must go to Williams for sticking with it, the statistics told something of a story, with Williams only hitting three winners in the set, but with Keys presenting her with 18 unforced errors.
It was Williams who grabbed the first break of the decider to take a 2-1 lead, but Keys would bring it back to 3-3 soon after. The pair then traded breaks before Keys held her nerve to secure her spot in the semi-finals.