Errani had won the pair's last three meetings in straight sets but there was something very different about this meeting, as Cornet - France's No.1 player now after Marion Bartoli's retirement - was particularly sharp, barely losing a 69-minute first set, winning three easy games to run away with the second set and battling back from 5-3 down to push the deciding set to a decisive tie-break.
Cornet's fighting spirit never waned as she again rallied from 5-3 down in the breaker to even it up at 5-all, but the No.3-seeded Errani snuck out two quick points to shut the door, 76(3) 36 76(5).
It was Cornet's second straight three-hour, two-minute marathon - in Friday's quarterfinals the former World No.11 took the exact same amount of time to beat Andrea Petkovic, a former World No.9.
Shortly afterwards, Errani withdrew from her doubles semifinal match alongside Roberta Vinci.
"I was very happy to win the semifinal today, but I'm sorry to have to withdraw from the doubles," Errani said. "But it was impossible for me. So that was the decision. I hope to recover for tomorrow."
The World No.7 was very complimentary in her press conference of Cornet's level, however.
"I'm not really surprised, because she's an unbelievable player, and we've played many times," Errani said. "She made a very good match today. It's great she could play so well here at home too."
Errani is now a win away from the biggest title of her career. She has been to three finals before at the Premier level or above - Roland Garros in 2012 (falling to Maria Sharapova) and this tournament and Dubai in 2013 (falling to Mona Barthel and Petra Kvitova). She is 7-7 overall in WTA finals.
Awaiting her in the final is Anastasia Pavlyuchenkova, who beat Maria Sharapova in the first semifinal of the day (read more here). Pavlyuchenkova beat Errani in the pair's only previous meeting in 2011.
"I know Pavlyuchenkova well," Errani said. "She's a very good player. Today she beat Maria playing incredible tennis, serving very well and fighting a lot on the court. I saw their match today.
"It will be tough. I have to be fully ready and playing well for tomorrow."