On Monday, Giorgi - who had come out of the qualies - upset No.4 seed Maria Sharapova in a center court thriller, her win over the No.5-ranked Russian representing the biggest win of her career so far.
But Giorgi came back down to Earth Tuesday, falling to No.20 seed Pennetta in 56 minutes, 62 61.
"She's so powerful - her backhand and forehand are really good - and she has really good potential," Pennetta said of Giorgi after the match. "But her game is completely unpredictable. You don't know what she's going to do - she can play really well in one point, but then not too much in the next point.
"If she can find the right balance she will be a great player."
Awaiting Pennetta in the quarterfinals is No.17 seed Sloane Stephens, who overcame some tense moments in the end of her match to close out Alisa Kleybanova, 63 75. Stephens led 63 42 when Kleybanova won three games in a row and served for the second set at 5-4 - Stephens regrouped.
"She played a good match. We both played pretty well," Stephens said. "It was my first time playing on Stadium 2. That was an interesting thing for me. It was pretty loud in there, but the court was good.
"I played solid; she played solid. I was happy to get the win in the end."
"No. I think at the beginning of the tournament is when you have the most nerves," Stephens said. "Then once you get going, you're like, 'The person I'm playing has done well, you're both playing well, so it's good we are this deep in the tournament.' But I think at the beginning of the tournament, first round, you're like, 'Oh my God, please don't lose first round.' You're worried about that a lot.
"I think once you get deep in the tournament you feel a lot better."
Pennetta leads Stephens in their head-to-head series, 2-1, though they've only played on clay.
"Yeah, should be good," Stephens said. "She just came back from an injury. She's been playing well since she's been back. It will be good. Obviously it's a big tournament; always going to be tough.
"I'm just going to go out and do my best and hopefully get the win."