Caroline Wozniacki beat American Sloane Stephens 7-6 (6), 6-2 on Thursday to reach the semifinals in New Haven for the sixth consecutive year.
The four-time champion is 23-1 at the Connecticut Tennis Center, with her lone loss coming a year ago when she had to retire from the semifinals because of a knee injury.
The former top ranked player will play Simona Halep after the Romanian beat Russia's Ekaterina Makarova 6-1, 7-6 (6).
The other semifinal will feature two players from the Czech Republic. Defending champion Petra Kvitova beat Russia's Anastasia Pavlyuchenkova 2-6, 6-2, 7-5. She will face Klara Zakapalova, who came from a set down to beat Russian Elena Vesnina 4-6, 6-0, 6-4.