The 24-year-old second seed from Belarus took only 76 minutes to defeat the 48th-ranked Slovakian while 83rd-rated Pennetta reached her first Grand Slam last four by ousting 10th-seeded compatriot Roberta Vinci 6-4, 6-1.
World number one Serena Williams, seeking her 17th Grand Slam title and fifth US Open crown, will meet Chinese fifth seed Li Na, the 2011 French Open champion, in Friday's other semi-final at Arthur Ashe Stadium.
“It’s great to see such great players out there and have the chance to play against them,” Azarenka said.
Photos: Shah Rukh Khan, Deepika Padukone, Alia Bhatt at an awards show
Photos: Big B, Iulia Vantur at Himesh Reshammiya's album launch
Photos: Aamir Khan with 'Dangal' actresses at fashion show
Spotted: Twinkle Khanna, Shriya Saran at an event in Mumbai
Andrew Flintoff's birthday: 10 players who entered different sports