The Czech left-hander, who has blasted into the world's top 10 this year, broke Pironkova's serve to love in the opening game of a first set she dominated.
The eighth seed had to battle back in the second as Pironkova, also a surprise semi-finalist last year, loosened up to establish a 4-2 lead but Kvitova responded to take the set into a tiebreak.
The 21-year-old Kvitova had her nose in front in the breaker but two consecutive forehands into the tramlines allowed her opponent to take the match to a decider.
Kvitova, bidding to become the first left-hander to win the women's singles at Wimbledon since Martina Navratilova in 1990, quickly put the setback behind her to dominate the third set, sealing victory with her ninth ace to set up a semi-final against Victoria Azarenka or Tamira Paszek.
"I'm very happy that I managed to break early in the third set and it was a different game then," Kvitova said in a televised interview.
"It's good I have the experience of last year when I got to the semi-final and I'm looking forward to playing Maria."
(Reporting by Martyn Herman, editing by Ed Osmond)