— -- NEW YORK -- With most of the focus on Serena Williams and the top half of the women's US Open draw over the past week and a half, the bottom half has enjoyed the benefits of operating under the radar.

What those players did not escape Wednesday afternoon, however, was New York's brutal heat and humidity, with one of only two women to have defeated Williams in 2015 wilting under the conditions, as No. 26 seed Flavia Pennetta outlasted No. 5 Petra Kvitova 4-6, 6-4, 6-2.

"It [was] everything," said Pennetta, advancing to her second Grand Slam semifinal. "I think it's everything, not just the heat [but] the tension for the match, what you think you have to be, what you want to be and to do. So many things on your mind, you try to take out everything and just play tennis."

With one women's semifinal already set for Thursday between Williams and 43rd-ranked Roberta Vinci, Pennetta became the second Italian in the semis [with Vinci] and now awaits the winner of Wednesday's final quarterfinal between 20th-seeded Victoria Azarenka and No. 2 Simona Halep.

Kvitova, 25, who had yet to drop a set in the tournament coming into the match, was aiming to become just the fifth active women's player to reach the semifinals or better at all four Grand Slams during her career.

Instead, with the extreme heat policy in effect allowing players to take a 10-minute break between the second and third sets and Kvitova getting slower as the match wore on, Pennetta, at 33, made her run in the second set when she ripped a backhand return for a winner to break Kvitova and take a 5-4 lead.

"I'm disappointed that I lost today for sure, but on the other side, I think I have a lot of positive things to take from the tournament," Kvitova said.

Pennetta, who converted only six of 16 break-point chances in the match, then broke at 3-2 in the final set, taking the last bit of fight Kvitova seemed to have in the process.

Among the more glaring discrepancies: Kvitova had 24 unforced errors on the backhand side to just two for Pennetta, and 26 on the forehand side to 10 for Pennetta.

It had been a disappointing year for Kvitova before the US Open, as the two-time Wimbledon champion (2011 and 2014) failed to get past the first week of the three prior majors.

A diagnosis of mononucleosis in early August explained her fatigue and lingering sore throat, and she has long said her asthma symptoms tend to flare up the worst when she is playing in Australia and the U.S. She also said she had to endure her longest match of the tournament Wednesday, at two hours, 23 minutes.

"I was kind of hot out there today," Kvitova said later. "But my energy-wise wasn't probably the best today, as well."

The heat and humidity surely did not help on Wednesday, long service games seemingly sapping her energy in both sets as she covered herself in ice at changeovers while Pennetta, eight years her senior, appeared not to be bothered.

"I'm sorry for her," Pennetta said. "It's not a good feeling when you feel so tired you can't move really well. She played a really tough match, running everywhere. She has to be proud of herself because she's playing until the last point."