The 31-year-old, ranked 83rd in the world, made it to her first grand slam semi-final with a win over fellow Italian Roberta Vinci.
The pair have enjoyed a friendship of 20 years - including a spell as doubles partners - so there were bittersweet feelings at the end of the routine 6-4, 6-1 win.When the final point was won, Vinci gave Pennetta a kiss on the cheek and told her, "Brava."
"It was tough for me of course," said Pennetta, who will face either Victoria Azarenka or Daniela Hantuchova for a spot in Sunday's final. "If you can choose, you want to be in the final, not against her directly. It's much better.
"We grew up together. I know how she plays; she knows everything of me. We get on the court, and I think today was more about inside, you know. It was not about tennis. It was about how you play, how you feel in the court, and how you can handle the situation.
"I'm proud though. I'm 31, and physically I feel good finally. I'm in the first grand slam semi-final."
This was Pennetta's fourth straight win over a seeded opponent - a spell that began with a 6-3, 6-1 win over another Italian, fourth-seeded Sara Errani, and also included a victory over world No.27 Svetlana Kuznetsova, the 2004 US Open champion. Even though Pennetta is yet to drop a set, Vinci, desperate to shed a tag of doubles specialist - she is ranked No.1 in the world with Errani - fancied her chances.
Trailing 5-4 in the first, she saved a pair of set points and had two chances to break, but couldn't convert either. She was visibly deflated and the second set lasted a mere 24 minutes.
"Yes, it was of course a special day for us," admitted Vinci. "It was a great chance to go to the semi-final. This time she won, but I think that I'm happy, too, about this tournament."