The Italian's run at Roland Garros last year, which was eventually ended by Maria Sharapova, was a surprise but it was no flash in the pan.
Errani is at a career-high ranking of fifth and two days ago she posted her first win over a top-five player by beating Agnieszka Radwanska.
Upsetting Williams will be a different task altogether, though. The world number one is on a 29-match winning streak and has lost just three of her last 75 matches dating back to a shock first-round exit in Paris last year.
The pair have met five times before and Errani has so far mustered a single set. Their most recent meeting came last month in Madrid, when the Italian matched her opponent for most of the first set.
At 5ft 4 1/2in, Errani is giving away a fair amount of height and a lot of power, and she said: "It's for sure very difficult because she's very strong.
"Physically she's an incredible athlete, so it is not easy to play against her because she has a lot of power. So it will be tough, but maybe on clay it's a bit better than the other surfaces."
Williams dropped her first set of the tournament in beating Svetlana Kuznetsova on Tuesday, and she is ready for another tough contest in her first French Open semi-final for a decade.
The top seed said: "It's going to be a really good match. You've got to appreciate how consistent she is. We had a pretty tough match in Madrid. I'm going to be ready.
"I'm glad I played her in Madrid because I know she's here to play and she's here to compete. She's so serious. I can be ready for that."
The first semi-final is, on paper, a much closer contest between defending champion Sharapova and third seed Victoria Azarenka.
The pair have met 12 times before, with the Belarusian leading 7-5, although Sharapova has won both their previous meetings on clay.
The Russian said: "She's playing really well this tournament. It's always a tough match. We have played each other so many times there are really no secrets between us in terms of our game styles and what we do well and not."
Azarenka has always found clay a struggle but she is feeling much happier about playing on the red stuff this season - not that it is a happy marriage just yet.
"I still don't have any ring on my finger," she said. "But I feel like we made a step forward.
"We are moving in together. The relationship is moving forward and we'll see what happens after."