Casey insisted he was back to full fitness after withdrawing from the defence of his Houston Open title last week after hurting his shoulder and neck in practice.
Fifth seed Dementieva, who won the Paris Indoor Open title on Sunday, suffered a shoulder injury in her match against Daniela Hantuchova and pulled out while trailing 6-4 1-1.
The 22-year-old Sharapova won her fifth consecutive match in straight sets, claiming her first title since triumphing in Tokyo in October last year as she continues her rehabilitation from long-term shoulder problems.
Sharapova, slowly working her way back to her best after a long-term shoulder injury, made a shock first round exit at the Australian Open, but is on course for her first title of 2010.