It's not a Grand Slam that's on the line, but both women are looking to get a shot at their first major title. Neither one has ever made it to a US Open final. Both expect a fight.
"To make a winner [against] her you have to finish the point seven times," Pennetta said of Halep.
Said Halep: "I know that she never gives up. ... I just have to play smarter than usual. I have to be aggressive."
Pennetta holds a 3-1 head-to-head advantage, including a 6-2, 7-6 (3) win over Halep in the fourth round of the 2013 US Open. Halep won their most recent meeting, though, on the hard courts of Miami earlier this summer.
Here's a look at some of the key numbers.