Williams had to rally from a set down in each of her last three matches, but it was a walk in the park for her on Day 11 of the fortnight.
She scalded 10 aces, compared to zero for Errani, who has recorded the weakest serve of this fortnight, while Williams has struck the hardest.
The reigning Australian and U.S. Open champion Williams owns a pair of French Open titles (2002, 2013).
The soft-hitting Italian Errani was the French Open runner-up in 2012 and played in her fourth straight French Open quarterfinal.
Williams, who's now 30-1 this year, is seeking her 20th Grand Slam singles titles. Only Margaret Court (24) and Steffi Graf (22) own more. Helen Wills Moody, like Williams, also owns 19 major championships, all in the Amateur Era.
Williams' semifinal opponent on Thursday will be 23rd-seeded Swiss Timea Bacsinszky or Belgian upstart Alison Van Uytvanck, who did not face a seeded player in her first four matches here.