Pennetta beat Halep, the world No. 2, in straight sets.
Flavia Pennetta advanced to the U.S. Open women's final with a 6-1, 6-3 upset over No. 2 Simona Halep on Friday.
Pennetta, the tournament's No. 26 seed, will play the winner of No. 1 Serena Williams and Roberta Vinci.
After dominating the first set, Pennetta broke Halep to start the second set. But the Romanian rallied, winning three straight games and appearing to find her game.
But Pennetta recaptured momentum, winning the last five games of the second set to advance to the final. Pennetta won 19 of the match's last 21 points.
"Today I play really well," Pennetta said. "I didn't make mistakes. I was focused. I was aggressive."
Pennetta upset No. 5 Petra Kvitova to reach the semifinals. Before this tournament, her best Grand Slam singles result was a quarterfinal appearance at the 2014 Australian Open.
The U.S. Open women's final is scheduled to be played at 3 p.m. ET on Saturday.