Kvitova je bila u ulozi pete nsiteljke turnira, a u meč je ušla bez izgubljenog seta. Ali, šampionku Vimbldona je izdalo telo, po vlažnom danu u Njujorku slavila je Italijanka rezultatom 4-6 6-4 6-2.
"I am really happy right now," Pennetta, the 26th seed, said on court afterwards. "I did not think I could get through this match. It's unbelievable."
Pennetta will now face either Simona Halep or Victoria Azarenka in the last four. She first reached the semi-finals at Flushing Meadows in 2013, losing on that occasion to Azarenka.
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The Italian flag will feature prominently in the semi-finals with the 33-year-old Pennetta joined in the last four by 32-year-old compatriot Roberta Vinci, who booked her spot on Tuesday with a victory over Frenchwoman Kristina Mladenovic.
Pennetta has always been at home on the New York hardcourts having now advanced to quarter-finals or beyond in six of her last seven visits to the National Tennis Center.
The same courts, however, have not been kind to Kvitova, the two-time Wimbledon champion, who never ventured past the fourth round until this year.
Kvitova, one only two players to beat Serena Williams this season, had been in superb form winning her final U.S. Open tuneup in New Haven and carried that momentum into Flushing Meadows, reaching the quarter-finals without dropping a set.
The Czech had looked well on her way to a first semi-final appearance when she easily took the opening but the veteran Pennetta kept her cool on another sizzling day in New York.
Flavia Pennetta sent a simple shot into the tramlines at set point, but still powered into the US Open semi-finals at the expense of Petra Kvitova.
Serving at 5-4 in the second Pennetta wobbled, missing an easy winner to hand Kvitova a break chance but the Italian did not buckle and hung on to level the match and then dominated the third set as Kvitova wilted in the oppressive heat.
"It's not just the heat, it's the tension for the match, for what do you think you have to be, what do you want to be, what do you want to do," said Pennetta.
"It's so many things in your mind. You just try to take everything out of your mind and just play tennis.
"I'm sorry for her. It's not a good feeling when you feel so tired and you cannot move very well.
"We played a really tough match, more than two hours running everywhere. She has to be proud of herself because she was playing up to the last point."
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