The American teenager, who came through qualifying, posted the most impressive of her victories in the quarter-finals yesterday against world No 16 Yanina Wickmayer. The Belgian third seed won the first set on a tie-break but Riske, 19, prevailed to win 6-7 (5/7), 6-4, 6-3.
Sharapova ended the challenge of another qualifier, Sesil Karatantcheva with a 6-2, 6-4 win, to reach her seventh successive semi-final in this event.
Three of the other four quarter-finalists were playing their second matches of the day, with bad weather having played havoc with the schedule.
France's fourth seed Aravane Rezai booked her last-four place following a 7-6, 7-5 win over Italy's Sara Errani and will play Chinese top seed Li Na, who beat Germany's Angelique Kerber 4-6, 6-3, 7-5.