The No.1-seeded Kvitova was the first of the two to move through, losing four of five games midway through the second set but recovering to make it past Misaki Doi in a tight two-setter, 64 64.
"It's always tough to play your first round at any tournament, and this time it's my first match of the year on clay," Kvitova said afterwards. "But I felt quite good on the clay here, I just need a few matches to really start feeling comfortable on it. But I'm very happy I was able to win in two sets today!"
The No.2-seeded Vinci followed Kvitova into the second round shortly afterwards, building a 62 20 lead against Andrea Hlavackova before the Czech had to retire from the match due to dizziness.
"I like it here," Vinci commented. "Everyone is really friendly and the court is quite fast, which is a good thing because it suits my game. I'm just taking it one at a time and we'll see how it goes."
And the upsets didn't stop there - playing her second round match, No.3 seed Klara Zakopalova was taken out, twice battling back from a break down to capture the second set but then letting go of a 3-1 lead in the deciding set to fall to tenacious Italian qualifier Maria Elena Camerin, 75 36 63.
Camerin's win over the World No.21 was the fifth-best win of her career. She has one Top 10 win, against No.2 Anastasia Myskina at Bali in 2004, and three more Top 20 wins, against No.18 Karolina Sprem at Hasselt in 2004, No.16 Nathalie Dechy at Rome in 2005 and No.18 Li Na at Berlin in 2007.
Kvitova and Vinci are actually the only two seeds left in the tournament - also among the fallen favorites so far this week was No.4 seed Julia Goerges, who had to retire due to dizziness against good friend Jill Craybas on Tuesday. The American qualifier expressed her sympathies afterwards.
"I played a friend today and it was really tough," Craybas said. "We train together a lot and we actually have the same coach. We've played practice matches against each other many times. So I think in the first set we were a little bit unsettled, but then I started playing a little bit better. It was tough.
"But I need to get some matches in and that's what I've been able to do here."
Next up for Craybas is Shahar Peer. "I haven't played her in a long time," Craybas said. "She's a former No.11 in the world so it'll be a tough match. She's a great competitor. It will be fun."
Earlier on Wednesday there was a minute of silence after the warm-up of the first match to remember the third anniversary of the Polish Air Force Tu-154 plane crash on April 10, 2010, that killed all 96 people on board - including Polish President Lech Kaczynski, his wife, Maria, and many others.