Caroline Wozniacki continued her impressive run at the New Haven Open - although the four-time champion survived a scare against Peng Shuai.
Wozniacki has lost just once at the event in six years however she was a set down to Peng.
The former world No 1 recovered in the second and the score was 6-2 1-3 when Peng was forced to withdraw through illness.
"It's a great tournament for me," said Wozniacki. "What else can I say?"
Sixth seed Sloane Stephens was a straight-sets winner, however Roberta Vinci and Dominika Cibulkova both suffered shock defeats.
Stephens saw off Anna Schmiedlova 6-4 6-4 and will now face Julia Goerges, but fifth-seeded Vinci went out 7-5 6-3 to Carla Suarez Navarro and No 8 seed Cibulkova crashed out Klara Zakopalova 3-6 6-3 6-3.
Navarro's reward is a second-round clash with Simona Halep, a 6-2 6-1 winner against Daniela Hantuchova.
Elena Vesnina, Monica Puig, Alison Riske, Karin Knapp, Anastasia Pavlyuchenkova and Ekaterina Makarova also registered first-round victories.
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