Pironkova, a tournament qualifier, battled past Errani 7-6 (7-2), 6-3 by getting seven breaks in 11 games receiving serve. Pironkova won despite taking just 52 percent of the points on serve and dropping her serve five times.
Errani was good on 82 percent of her first serves but won only half of the points in those situations. Pironkova had 16 break chances in the match, finishing off three in the first set and four in the second.
No. 2-seeded Petra Kvitova beat Lucie Safarova 7-6 (7-4), 6-2 with eight aces and saving both break points she faced.
Also Wednesday, Madison Keys advanced when, after Keys was up 3-2 in the first set, Bethanie Mattek-Sands retired.
Keys draws Kerber in the second semifinal Thursday. Kerber was a straight-set winner in the fourth round at last year's Australian Open in their only previous meeting. Kvitova is 3-0 in her career versus Pironkova.