Basically, Serena just has to play her game and execute.
She was never consistent and agressive off the ground in 2004 like she was in 02-03, But she seems to have found her ground game back. When Serena's game is on, she hits alot more winners per match than even Sharapova, and such has been the case in this tournament.
When Serena is playing well off the ground, she can outplay and outhit Maria with power tennis from the baseline (just look at thte first 4 games of the YEC 3rd set) Rena just blasted winner after winner past Maria who looked defensless, until her injured serve got the best of her.
That being said, I still don't think she should only stay back, even if she is outplaying Sharapova from the back court. Serena volleys alot better than Maria, so she should come to net behind good approaches, and if her serve is on, then its bye bye Sharapova.