Twelve months ago, Muguruza made her Grand Slam breakthrough in Paris, knocking out reigning champion Serena Williams en route to the last eight. And on her return, the young Spaniard has barely put a foot wrong, making her way through the draw with the minimum of fuss.
Her fourth round encounter with Pennetta had originally been scheduled for Sunday, only to fall foul of the Parisian weather. However, the delay did not seem to affect Muguruza, who reeled off the final five games to complete a 6-3, 6-4 triumph.
"I think that in big tournaments I feel more motivated, and I can play my best tennis," Muguruza said. "I'm just really happy to be in the quarterfinals again here in Roland Garros. I think it's going to be my favorite Grand Slam. I'm just really excited."
Muguruza made an inauspicious start, dropping her serve in the opening game. She was not trailing for long, unleashing a backhand to restore parity before a flurry of further winners edged her 5-3 ahead.
After comfortably serving out the set, the No.21 seed suffered a lapse in concentration at the start of the second. A string of errors, capped off by a double fault, saw her slip an early break behind, but, just as in the first set, she came roaring back to set up a meeting with Lucie Safarova.
"I was in the locker room just looking at the last few points, and I saw that Safarova won," Muguruza said. "I have never played against her. You know, it's the first time we meet. But she's playing good. She's lefty and that gives her a lot of help. She's playing great. That's what I saw. I didn't think a lot more."
Muguruza is bidding to become the first Spaniard since 1998 to lift the women's title at Roland Garros. And in the build up to her meeting with Pennetta, the 21-year-old picked the brains of the woman whose achievement she is looking to emulate: new WTA Finals ambassador Arantxa Sánchez-Vicario.
"The other day I was talking to her in the locker room. She was giving me some advice, and you have to listen to a champion like Arantxa," Muguruza said. "I was like, 'Yes, tell me everything!'
"She was explaining to me about when she was my age, saying that you have to control your life outside the tennis, because it's also very important. She was telling me about that. On the court is where you know what to do, but the problems come from outside. It was just like the typical advice I would get from my mother!"