The world number one, a Roland Garros champion in 2002 and 2013, had been troubled in her three previous rounds, but a 6-1 6-3 crushing of the Italian claycourt ace showed the American was still a clear favourite to claim another Suzanne Lenglen Cup.
Williams will now face the 23rd-seeded Swiss Timea Bacsinszky, who beat Belgian sensation Alison van Uytvanck 6-4 7-5, for a place in Saturday's final, where either 2008 champion Ana Ivanovic or Czech Lucie Safarova await.
Having lost the first set in her three previous matches at Roland Garros, Williams was hoping to get off to a better start which she did, setting the tone by breaking in Errani's first service game.
A brilliant backhand passing shot handed Errani an immediate break back but Williams was in the groove and dominated the rest of the match.
"I definitely felt more in control, and I felt I had to be again, going up against a player who plays well on this surface in particular," said Williams, who has only been in three previous semi-finals at the French Open.
The American, however, says she is not feeling the strain of chasing her 20th major singles title.
"I'm not putting too much pressure on myself... I'm surprised I'm still in the tournament, so this is really cool for me," she said.
Williams would not have expected to be facing a semi-final against baseliner Bacsinszky, who became the first Swiss woman since Martina Hingis in 2001 to reach the last four at Roland Garros.
"For me, it doesn't matter who I'm going to be playing against. I just want to have one or two or three tactics and try to put them in the game," Bacsinszky said.
On the other side of the draw, left-hander Safarova, seeded 13th, will start as favourite against seventh seed Ivanovic.
Safarova's flat, powerful forehand proved too much too hot to handle for 2014 champion Maria Sharapova in the fourth round.
Ivanovic, however, has matured since winning here in 2008 and seems to have gained in composure.
(Reporting by Julien Pretot; Editing by Toby Davis)
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