The Paraguayan recovered strongly from being blanked out of the second set, completing a 6-3, 0-6, 6-4 comeback to book her place into her first WTA quarterfinal since June.
"Petkovic is a great player, she's tall and so she has a great serve," Cepede Royg told press afterward. "It was a tough match, but I've had tough matches before and I think I know how to play against the top players.
"I had to do my game, be aggressive and move. And I think I did."
The No.129-ranked Cepede Royg opened the match with a break, playing confident and aggressive tennis against the former Top 10 German.
But Petkovic found her groove in the second set, cutting down on the unforced errors and dictating play with her backhand. And as the German's lead grew, the Paraguayan's confidence seemed to diminish, responding passively and second-guessing herself.
"It's not easy to see the score at 6-0," Cepede Royg said. "But my coach came and said to just relax, enjoy this moment, do your game and fight for every point."
"You have to change your mentality. I was thinking too much in the match, so I focused more in my game."
With that renewed mindset, the World No.129 turned it around in the decider and broke back to stop Petkovic's run at 1-1 and keep the German under pressure.
They stayed on serve until a marathon final game, which saw Petkovic save four match points to keep herself alive but couldn't serve it out. The Paraguayan eventually prevailed on the fifth time of asking, finally breaking the Petkovic serve to take the match and a spot in the quarterfinals.
"Oh man, I was thinking, 'Oh my god, I've had five match points!'" Cepede Royg laughed. "I tried to be more aggressive and not think too much, just put the ball in the court and go for it. It was so difficult but I did it."
The victory, her first WTA main draw win in almost six months, puts her through to the second quarterfinal of her career since last year's Mallorca Open. She'll play the winner between Australia's Lizette Cabrera and Croatian qualifier Jana Fett for a spot in the semifinals.