The No.6-seeded Wozniacki looked in control early on in the match, keeping her unforced errors down to just six en route to taking the first set, 6-4. But that strategy became more and more ineffective against an increasingly aggressive Giorgi, who began teeing off on Wozniacki's serves and groundies late in the second set, holding at love then breaking at love to win it, 6-4, and send it to a third.
Giorgi kept swinging away in the third set, storming ahead 5-2 and bringing up three match points in that game - Wozniacki saved them all and ended up closing to 5-3, but Giorgi regrouped and served out the match, finishing off the two-hour, 32-minute, 46 64 63 triumph with a huge forehand winner.
"I loved playing on that court. My goal was to be there, and it was amazing to play there," Giorgi said. "Even if my first set wasn't really good, I stayed really focused, just focusing on every point after that. And even if I did mistakes, it's because I was trying to play my game out there, and my tactics."
And what were her tactics? "Just when the ball came, to hit the ball in the corners!"
Giorgi hadn't played a tournament since Wimbledon due to a shoulder injury, but she's clearly back.
"The nine months before Wimbledon you can't really count because I was playing every tournament injured, so I stopped for two months after Wimbledon this year," Giorgi said. "So this was my first tournament back, and I feel good now. My shoulder is good. So I'm really happy about everything now."
The match numbers said it all. Giorgi hit 46 winners to 45 unforced errors during the match, while Wozniacki was a more conservative - and much more negative - 13 winners to 27 unforced errors.
"I had many break point opportunities in the second set and didn't take them," Wozniacki said. "Then all of a sudden she came into the rhythm and started hitting everything and it started to go in. She put me under pressure and I started to play a little short, and all of a sudden she was all over the ball."
Giorgi, a No.136-ranked qualifier into the draw, is now into the second week of a Grand Slam for the second time. She also made the fourth round of Wimbledon last year - as a No.145-ranked qualifier.
Having never reached the second week in her first 30 Grand Slams, Vinci has now reached the second week at five of her last six Grand Slams, her only pre-fourth round loss at a major in that stretch coming at this year's Australian Open (and she actually held match point in her third round loss there).
Vinci and Giorgi will be playing against each other for the first time.