Romanian second seed Simona Halep advanced to her first US Open semi-final by defeating two-time Australian Open champion Victoria Azarenka of Belarus 6-3 4-6 6-4.
Halep advanced to a Thursday (Friday NZT) semi-final against Italy's 26th-ranked Flavia Pennetta, who upset Czech fifth seed Petra Kvitova, a two-time Wimbledon winner 4-6 6-4 6-2 in another quarter-final at Arthur Ashe Stadium.
Pennetta has a 3-1 career edge in matches against Halep.
"I'm really happy right now," said Pennetta, who ended Sam Stosur's tournament in the fourth round.
"I was just trying to fight every ball, running and push everything I can.
"I didn't think to get through this match. In the second set I was playing and playing, it was unbelievable."
World No.1 Serena Williams, who holds all four major titles and seeks a calendar Grand Slam, will face Italy's 43rd-ranked Roberta Vinci in the other semi-final.
Williams is chasing the first calendar Grand Slam since Steffi Graf in 1988 and trying to match Graf's Open Era career record of 22 Slam singles titles, two shy of Margaret Court's all-time record.
AFP