Top seeds Agnieszka Radwanska and Dominika Cibulkova booked their semi-final berths Thursday at the WTA hardcourt tournament in New Haven, the final tuneup for the US Open.
Poland's Radwanska, the defending champion, defeated eighth-seeded Peng Shuai of China 7-5, 6-3 to advance to the final four.
Second-seeded Cibulkova, of Slovakia, beat sixth-seeded Anastasia Pavlyuchenkova of Russia 7-5, 6-4.
Peng and Radwanska had split six prior encounters, with Peng taking their most recent meeting at Indian Wells in March.
But Radwanska, still seeking her first title of the year, emerged with a victory in just under two hours to set up a semi-final meeting with Australian Daria Gavrilova.
Gavrilova defeated Belgian qualifier Kirsten Flipkens 6-2, 6-4.
It was the third meeting of the year between Cibulkova and Pavlyuchenkova, who had split two prior contests in 2017.
Having narrowly won the opening set, Cibulkova powered to a 5-0 lead in the second, then turned back a furious rally from the Russian to win in just over two hours.
"She made it extremely tough for me in the end," Cibulkova said. "I was playing very well in the whole second set, but then she started playing even better.
"I had four match points, and she hit an ace every time so I couldn't do much about it. But I just kept fighting and I said to myself, 'OK, this one's going to be yours.'"
Cibulkova next faces Belgian qualifier Elise Mertens, who reached the semi-finals on a walkover when China's Zhang Shuai withdrew with a right arm injury.
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