The 20-year-old German credited her thrashing of Italian 10th seed Sara Errani 6-2 6-2 at the French Open to being "in a real zone".
"I didn't think at all. I was just focused on what I had to do. Mentally I was really good," she told reporters.
"When it was 6-2, 5-2 I sat down and I said to myself, Okay, Andrea, are you going to get tight now? And I paused and I said, No, not now... "I was just making my hands faster and then there was more rotation on the ball and it went higher over the net. So that was the only thing I was occupied with in my head."
Petkovic, seeded 28, was quicker to hit her stride in chilly, gusty conditions in Paris and despite losing her first service game she was the more aggressive player throughout.
Neither player could hold serve for the first four games of the second set, but she kept up her game plan, moving the 10th- seeded Errani, runner-up on the Paris clay in 2012, around the court.
Despite her delight at going further in a major than ever before - shown by her kissing her racket as victory sunk in - doubts are never far from her mind after being plagued by injuries which forced her out of the tour for several months over the past two years.
"Every time I have happy moments and nice wins, it immediately brings the doubt and the fear of that it might slip away again," Petkovic said.
"It's a difficult thing for me being all the way positive again and just trusting that everything will be fine, because I learned it the other way."
Her next test will come against Simona Halep, the wily Romanian fourth seed who outfoxed former French Open champion Svetlana Kuznetsova of Russia 6-2 6-2.
As for those philosophers, she said: "I would like to call Freud and ask him what he thinks about me and my twisted soul."
(Editing by Ed Osmond)
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