Williams' win was the most lopsided women's semifinal at Roland Garros since 1984, when Chris Evert beat Camille Benjamin 6-0, 6-0.
Sharapova beat Errani in the final last year to complete a career Grand Slam, but she faces a tougher test this time. She's 2-13 against Williams, who has been on a mission after more than a decade of disappointment in Paris.
"Obviously whatever I did in the past hasn't worked," Sharapova said. "So I'll have to try to do something different and hopefully it will."
The top-ranked Williams, a 15-time Grand Slam champion, won her only Roland Garros title 11 years ago by beating her sister Venus in the final.
"I'm very happy to be back in the French Open final 11 years later," Williams told the crowd in French. "I'm still here 11 years later. It's so wonderful for me."
While Williams easily won her semifinal, Sharapova advanced past Azarenka with a clamor. The two most notorious grunters in tennis wailed on nearly every swing, matching pitch and volume as they swapped powerful shots from the baseline. They sounded as if they were pushing a stalled Peugeot across lanes of traffic in the Arc de Triomphe.
"Come on, Monica," a spectator yelled at Sharapova, referring to one of the game's great grunters, Monica Seles.
The aggressive swings resulted in a seesaw semifinal. Sharapova whacked 12 aces but was erratic with her second serve, and her groundstrokes were also unpredictable.
She needed five match points to seal the victory. Serving for the victory for the second time, she held at love and finished with an ace.
"Those last few points are the toughest," Sharapova said. I'm so happy that I regrouped and came out at 5-4 and served it out really well."
Following a 35-minute rain delay before the third set, Sharapova hit four double-faults in a single game, the last of them on break point, to make it 2-all. She struggled again with her serve at 5-2, losing a tense, sloppy 12-minute game when she squandered four match points and double-faulted on the final two points.
She was steadier at the end, however, and after accepting a cursory congratulatory handshake from Azarenka, Sharapova screamed through a grin.
"To come back as the defending champion, it's extremely special to get back on that stage where it comes down to the last two players of the tournament," Sharapova said.
R Maria Sharapova beats Victoria Azarenka 6-1, 2-6, 6-4 despite 11 double-faults, and Serena Williams tops Sara Errani 6-0, 6-1 to set up the final match.
• Williams' win is the most lopsided women's semifinal since 1984.