Nineteen-year-old Riske qualified for the tournament having never previously won a main-draw match on the WTA Tour, and she claimed a major scalp when she knocked out Belgium's Yanina Wickmayer in the quarter-final.
But the American's hopes of advancing to a maiden Tour final were jolted when her Russian opponent eased through the first set 6-2. She launched a spirited effort in the second set and edged it 6-4.
But that was as good as it got for Riske as Sharapova took the deciding set 6-1. She will meet China's Li Na, the top seed, in today's final.