Pennetta has been in the main draw of major tournaments consistently since 2003 but only three times before this year reached quarterfinals. A win Wednesday over No. 10-seeded Roberta Vinci gave Pennetta her first spot in a major semifinal. She is just the 13th unseeded player to reach a major tournament semifinal since 2001.
She'll go against No. 2-seeded Victoria Azarenka in Friday's semifinals. The winner of that pairing moves into Sunday's title match against either No. 1-ranked Serena Williams, the defending U.S. Open champion, or fifth-seeded Li Na.
Williams has 16 Grand Slam titles -- four at the U.S. Open -- and is in 20-3 in major tournament semifinals. Li is 3-1 in previous semis and won the French Open in 2011. Azarenka is 3-3 in major semifinals, finishing second to Williams at the U.S. Open last year and having won the last two Australian Open titles. All overshadow Pennetta's resume.
Pennetta's path to the 2013 U.S. Open semifinals, however, is strewn with higher-ranked opponents. Pennetta, ranked as high as 10th in 2009 but now 85th, defeated fourth-seeded Sara Errani in the second round, topped No. 27 Svetlana Kuznetsova in the third, beat 21st-seeded Simona Halep in the fourth and ousted Vinci in the quarters. She is yet to lose a set.
She and Azarenka have split their two previous career meetings but Azarenka is 41-4 this year with three titles. Before the U.S. Open, Pennetta, slowed by a wrist injury last year, was 16-17 in 2013 and without a title since April 2010.
"Her ranking doesn't matter," Azarenka said of Pennetta. "It's about the moment right now and she's playing terrific tennis."
Li faces a formidable challenge in Williams, who is 65-4 on the year with eight titles (including a French Open championship). Williams is 8-1 all-time versus Li but both matches this year were close.
However, none of Williams' matches at the U.S. Open have been. She's dropped 13 games, a run punctuated by the 6-0, 6-0 rout of 18th-seeded Carla Suarez Navarro in the quarterfinals. Li's only dropped set in the tournament came in a tiebreaker.
"We've always played really close," Williams said of playing Li. "Maybe next time she might want to go from close to a win. I have to be ready for that."