New York - Italy's 26th-ranked Flavia Pennetta shocked two-time Wimbledon champion and Czech fifth seed Petra Kvitova 4-6, 6-4, 6-2 Wednesday to reach her second US Open semi-final.
Pennetta next will play Thursday against the winner of a later match between two-time Australian Open champion Victoria Azarenka, the 20th seed from Belarus, and Romanian second seed Simona Halep for a berth in Saturday's final.
Pennetta, 3-1 all-time against Haelp and 2-2 against Azarenka, matched her best Grand Slam run from the 2013 US Open semi-finals, where she lost to Azarenka.
"I'm really happy right now," Pennetta said. "I was just trying to fight every ball, running and push everything I can.
"I didn't think to get through this match. In the second set I was playing and playing -- it was unbelievable."
World number one Serena Williams, who holds all four major titles and seeks a calendar Grand Slam, will face Italy's 43rd-ranked Roberta Vinci in the other semi-final.
Williams is chasing the first calendar Grand Slam since Steffi Graf in 1988 and trying to match Graf's Open Era career record of 22 Slam singles titles, two shy of Margaret Court's all-time record.
Three-time defending champion Williams, seeking her seventh US Open crown, has a head-to-head edge on all possible finals foes -- 7-0 over Pennetta, 6-1 over Halep and 17-3 over Azarenka plus a 4-0 record against Vinci.
Kvitova's ouster removes a potential threat to Williams that delivered one of only two Williams losses this year when they met in May on Madrid clay.
Kvitova, the first Czech woman in the US Open's last eight since Daja Bedanova in 2001, made 60 unforced errors to only 16 for Pennetta.
Kvitova also had nine double faults against only four aces in hot conditions at Arthur Ashe Stadium.
"It's not just the heat," Pennetta said. "It's the tension for the match, for what you think, what you have to be, what you want to be, have to do -- it's everything in your mind. You have to take it out and just play."
Pennetta, 33, improved to 4-3 against left-hander Kvitova, 25, the 2011 and 2014 Wimbledon champion whose run was still her best at a US Open.
Kvitova, who had dropped only one service game in four matches, was broken at the start but broke back in the next game and rolled to a 5-2 lead.
Kvitova, her upper right leg taped, struggled serving for the set, double faulting on set point and to surrender a break chance, and Pennetta smacked a backhand winner to pull within 5-4, only to double fault away the set herself in the next game.
After an early exchange of breaks in the second set, Pennetta broke on a backhand service return winner for a 5-4 lead and fought off two break points to hold in the 10th game to force a third set.
Kvitova smacked a forehand long to hand Pennetta a break and a 3-2 lead, broke again in the seventh game and won after two hours and 23 minutes when Kvitova hit a forehand long.
Azarenka, who lost to Williams in the 2012 and 2013 US Open finals, is 7-7 in Slam quarter-finals while Halep, whose best Slam showing was a runner-up effort at last year's French Open, is 2-2.
Azarenka, 26, beat Halep, 23, twice in 2012 in their only prior matchups but Halep has won three titles this year while Azarenka has been nagged by a thigh injury.
Halep is on her deepest US Open run in her sixth appearance. She is the first Romanian in the US Open last eight since Irina Spirlea reached the 1997 semi-finals.