The 21st-ranked Italian, who won this event in 2009 and was the runner-up last year, will play Polona Hercog of Slovenia. Hercog earned her first career title in Bastad, Sweden, last week.
Four-time champion Anabel Medina Garrigues of Spain and Petra Cetkovska of the Czech Republic will meet in the other semifinal match.
Pennetta lost her opening service game against Pironkova but then dominated the match.
Pennetta broke the 49th-ranked Bulgarian three times to take the first set. She added two more breaks in the second and closed out the win when Pironkova netted a forehand.
The 20-year-old Hercog extended her winning streak on the WTA circuit to eight by routing fourth-seeded Klara Zakopalova of the Czech Republic 6-1, 6-1. Hercog converted six of her seven break points.
Medina Garrigues defeated Irina-Camelia Begu of Romania 6-2, 1-6, 6-0. The 39th-ranked Spaniard, who lost to Begu in the semifinals in Hungary last week, wrapped up the victory on her second match point.
Medina Garrigues will take on the 62nd-ranked Cetkovska, who rallied to edge the third-seeded 2008 champion Sara Errani 5-7, 6-3, 7-6 (4) and reach her first career semifinal.