The withdrawal of Flavia Pennetta today allowed US Open top seed Caroline Wozniacki to reach the last four of the Pilot Pen Tennis without playing in New Haven, Connecticut.
The Italian's decision to pull out with a foot injury handed Wozniacki, who won the Rogers Cup in Montreal on Monday, the US Open Series title.
The Dane will face Elena Dementieva in the semi-finals after the Russian beat Marion Bartoli 6-3 3-6 6-2.
Nadia Petrova beat French Open finalist Sam Stosur 6-2 6-1 to set up a meeting with Maria Kirilenko, who defeated Dinara Safina 6-3 6-3.
Sergiy Stakhovsky ended Marcos Baghdatis hopes of victory in the men's quarter-finals with a 5-7 6-1 7-6 (7/4) victory.
Top seed Baghdatis, a semi-finalist in Cincinnati last week, secured the only break of the first set to take it 7-5 but his Ukrainian opponent dominated the second set as he lost just one game.
That set up a tense decider and, despite each man earning a break of serve, it eventually went to a tie-break with Stakhovsky holding his nerve to reach the last four.
Stakhovsky will now meet Thiemo De Bakker in the semis after the Dutch 12th seed claimed a comfortable 6-4 6-3 win over Evgeny Korolev.
The other semi-final sees Denis Istomin meet Viktor Troicki.
Istomin came through a final-set tie-break to defeat Teymuraz Gabashvili 6-1 3-6 7-6 (10/8) while Troicki beat Radek Stepanek 6-2 6-3.