Williams, seeded sixth, ousted the No. 4 seed Kvitova under the roof on Center Court at the All England Lawn Tennis Club to set up a meeting with second-seeded former world No. 1 Victoria Azarenka.
The 31-year-old American took 84 minutes to top her Czech opponent, employing a dominating serve. Williams' performance included 13 aces -- including three in the final game -- and no double faults. She allowed Kvitova only one break-point chance.
"I'm going to have to [step it up again] if I'm going to win the title," Williams said afterward. "There's a lot of people vying for it still."
Azarenka, the Australian Open champ, got past game Austrian Tamira Paszek 6-3, 7-6 (7-4). The Belarusian will be making her third major semifinal appearance after going 1-1 in her previous two.
Also reaching the semis was eighth-seeded German Angelique Kerber, who eliminated her 15th-seeded compatriot Sabine Lisicki 6-3, 6-7 (7-9), 7-5. Kerber needed five match points to put away Lisicki, who upset world No. 1 Maria Sharapova Monday.
Kerber will appear in her first Wimbledon and second career Grand Slam semifinal, facing the Agnieszka Radwanska-Maria Kirilenko winner Thursday.
Also Tuesday, third-seeded Pole Agnieszka Radwanska bested 17th-seeded Russian Maria Kirilenko 7-5, 4-6, 7-5 in a match that began at Centre Court and was forced indoors due to rain.
The 23-year-old Radwanska, who has a shot at claiming the No. 1 ranking this week, will appear in her first-ever Grand Slam semifinal.