Shock defeat for Halep
Top seed Simona Halep crashed out of the New Haven Open after a three-set loss to Magdalena Rybarikova.
Defending champion Halep was left to rue missed chances as she suffered a 6-2 4-6 6-3 loss in Connecticut.
Halep took just four of 14 break opportunities, while Rybarikova pounced to break the Romanian on six occasions.
There was another upset as unseeded Alison Riske toppled No 6-ranked Flavia Pennetta 6-1 7-6 (7-3).
"There are a lot of top players here. I'm definitely proud to be the lone American left, and hopefully I can keep it rolling," said Riske, who will now face Rybarikova.
"I knew that I had to attack her second serve because she places her first serve so well. I felt like I was going to get looks at it at any moment, so I just kind of stuck with it, and I ended up returning well today. I just played solid."
Second-seeded Petra Kvitova, who lost to Halep in last year's final, needed just 48 minutes to brush aside Ekaterina Makarova 6-2 6-1, while Barbora Zahlavova Strycova eased past Caroline Garcia 7-5 6-2.