Though the No.21-seeded Halep had won her first four WTA titles and had rocketed up the rankings this summer, and had also recorded her best Grand Slam showing by reaching the fourth round, Pennetta had a lot of things in her favor going into the match too - she had already taken out No.4 seed Sara Errani and No.27 seed Svetlana Kuznetsova here, she had been to the quarterfinals here three times before, and - perhaps most important of all - she had beaten Halep in both previous meetings.
Perhaps buoyed by all that, Pennetta came out very strong against Halep, storming out to a 62 42 lead. Halep found her range from there, winning three straight games to go up 5-4, and even got to set point at 40-30 in that game, but that's when another storm came - unfortunately, a New York rain storm.
Several hours later, around 5 o'clock, the two players took Louis Armstrong Stadium again, and Pennetta saved that set point and broke back for 5-all; she dropped serve in the next game but broke back again on a huge forehand down the line winner, sending the match to a second set tie-break.
Pennetta raced out to a 6-1 lead in the tie-break and, after two errors, she finally closed it out, blasting a big backhand down the line winner from behind the baseline to seal a 62 76(3) decision.
This is Pennetta's fourth Grand Slam quarterfinal, and all of them have come here - 2008, 2009, 2011 and now 2013. But this one might be the most special - it's her first since her 2012 wrist surgery.
"It was and still is really tough," Pennetta said of her comeback. "A lot of things changed when I came back in March. My coach, my doubles partner, the feelings I had on the court, everything. But I just tried to come back and play as many matches as I could, singles, doubles, mixed, everything.
"When you stop for such a long time, like seven months, you feel different. You might not have a problem in your hand, but you start having problems in your leg or abdominal muscle or something, because you aren't used to playing. Your body needs more time to recover and it's really difficult.
"My results since I came back this year have been up and down. But I was always working hard, still working every day with my coach, with my physio, with the physical coach and my psychologist, too.
"It hasn't been easy, I can tell you that. I've cried a lot. But I love this sport. I tried my best - if I could come back, good, but if not, I had a good career and didn't have to do anything more. But I wanted another chance to prove myself. I tried 100%, and I did it. And now, my results are good again."
There must be something special about the US Open, too. "Playing in New York is always good for me," Pennetta said. "I really love this place. I have good memories. But it's hard to say why. I just come here and practice hard and try my best all the time every match, and it has just happened this way.
"I can tell you that on the first day this year, when I played my first match, I didn't expect to be here. I didn't expect to play so well against Sara in my second match, either. But I've just tried to play my best all the time and be aggressive, do something with the ball and not think about winning or losing.
"Just try to play tennis."
Next up for Pennetta is No.10 seed Roberta Vinci, who won her fourth round match earlier in the day, just before the rain came (read more here). Vinci and Pennetta are tied in their head-to-head, 4-4.