Chasing a 50th career title at the clay event in the Spanish capital, defending champion Williams was far too strong for Italian seventh seed Errani on the Manolo Santana show court, winning 7-5 6-2 before Sharapova made short work of 16th seed Ana Ivanovic of Serbia to win their semi-final 6-4 6-3.
The Russian second seed and world number two, 26, will replace 31-year-old American Williams at the top of the rankings if she gets the better of her in Sunday's title match.
Williams, whose interests away from tennis include fashion and acting, said she had been avoiding "too many extra-curricular activities" to concentrate on match preparation.
"I usually just stay in my room really and be the most boring person you can ever imagine," she told a news conference. "That's kind of the way I get focused."
Sharapova will be a different proposition to Errani as the French Open champion, who beat the Italian in last year's Roland Garros final, is among the players on the Tour who can come close to matching Williams for power.
However, she has only beaten the American twice in 14 attempts, and never on clay, and both those victories came almost a decade ago.
"I look forward to it," Williams said.
"I feel like this whole tournament I've only played clay court players from my first round to now.
"Everyone was also smaller than me. So I think tomorrow will be a really good match. Different game. More power obviously, but still a lot of the consistency."
Sharapova said she needed to be more consistent if she was to have any hope of beating Williams.
They last played in Miami this year when Sharapova won the first set before going down 4-6 6-3 6-0.
"I thought I played really well, at a good level, in Miami for the first set and a half but that's not enough," she said.
"Obviously the goal is to keep that level for the whole match and to take my opportunities.
"I haven't had a win against her in a long time. But the great thing is that I'm setting myself up in a position where I can try to change that around."