Australian open/Melbourne:The unseeded 19-year-old came through 6-3 4-6 6-4 in one hourand 54 minutes.
The American goes on to face top seed Serena Williams, whoswept past 11th seed Dominika Cibulkova 6-2 6-2.
Defending champion Stan Wawrinka beat Kei Nishikori 6-36-4 7-6 (8-6). Novak Djokovic plays later on Wednesday.
Keys overcame a recurrence of the upper leg injury thathampered her at Wimbledon last year, requiring a medical timeout off courtduring the second set.
"It was definitely kind of a flashback to Wimbledonfor me," she said. "I have had some problems with that part of myleg, so it was kind of an overwhelming moment.
"It was kind of scary but luckily I was able to catchit before I did any real damage to it."
Serena Williams fired down 15 aces in a comprehensive 65-minutewin over last year's finalist Cibulkova.
The 33-year-old American has won the title on all fiveoccasions she has made it past the quarter-finals.