Petkovic trailed No.7 seed Eugenie Bouchard, 62 41, in her opener but surrendered just two games the rest of the way, in fact winning 16 straight points late in the second set and early in the third en route to a 26 75 61 victory.
"I really felt like she was playing great, playing really aggressively, taking the ball early, taking time away from me, and I wasn't moving my feet at all," Petkovic said. "She put so much pressure on me. What I really tried to do and what I'm better at now than when I was younger is I tried to stay in the moment. I do meditation every day to get in the moment and stay in the moment when it's going downhill and I somehow managed to do that. It wasn't easy at all and I think that's the only way to get out of that kind of situation. And the audience also helped me turn it around."
Petkovic said her recent injury struggles actually benefitted her in Wednesday's comeback.
"Even a year ago I wouldn't be able to do that," Petkovic said. "I feel more relaxed because I think there's something much worse that happened to me last year. I'm just kind of happy to be there and playing instead of lying in bed and having surgeries and going through rehab. I think subconsciously I'm thankful for what I have and it makes me calmer than I used to be before my injuries."
As for Beck, she won four straight games from 4-2 down in the third set for a 62 46 64 defeat of fifth-seeded Lucie Safarova and a quarterfinal berth.
"I had a tough match against Lucie," Beck said. "I had a great start, was hitting the ball great and was using my patience to get her under pressure. She got stronger in the second set and it got pretty tight at the end. I just kept fighting for everything and I'm very happy I came through."
"She's had some good tournaments recently and is on a run," Beck said. "She's got a lot of self confidence, but so do I after my win. I'm happy that I have a day off so I can prepare for that match."