There were some tricky moments for the No.1-seeded Williams early on - she needed a few deuces to hold her opening service game, then got broken in her second one to trail 2-1; but she took complete control from there, reeling off 11 of the next 12 games to dismiss the Romanian qualifier, 63 60.
"I don't know if I feel better on clay this year or last year," Williams said. "I felt great last year and I feel great this year too. I love playing on clay and have done very well on it, even at Roland Garros."
After rolling through the first set without losing a game - she didn't even let Errani get any game points, in fact she only let her have seven points total - Azarenka was really made to work in the second set, losing four of five games at one point and having to break back as Errani served for it at 5-4. But she finally closed out the one-hour, 14-minute second set to move through to the Premier-level final.
"I know the score looks easy in the first set, but it was not easy to play," Azarenka said. "And in the second set I was happy with the way I played but I didn't take all my chances. I lost focus and Sara is a great fighter and started to play hard and go for her shots, so I had to change my game a little bit. I think I could have done more in the second set but she's a great player and I'm happy to win.
"Over the last couple of years clay is the most challenging surface for me, but I'm feeling more and more comfortable on it and this week is proof. I'm finding my game on this surface right now."
Williams leads Azarenka in their head-to-head series, 11-2, and did win their only previous clay court meeting handily, a 61 63 victory in the final of Madrid last year. But Azarenka holds one very relevant advantage - she won their last meeting, their only 2013 meeting, in the final of Doha this year.
"She played well in Doha," Williams said. "I had opportunities I wasn't able to use and Vika played well in the important points. It was a tough tournament, though, and I was happy to be in the final."
"I'm excited about tomorrow," Azarenka said. "It will be great to play someone at that level."