Sara Errani's U.S. Open preparations were jolted on Wednesday when the top-seeded Italian was upset by Russia's Ekaterina Makarova 7-5 6-1 in the second round of the New Haven Open.
World number five Errani double-faulted five times while her 26th-ranked opponent unleashed eight aces and won seven consecutive games on the way to victory in one hour 22 minutes.
The premature exit of Errani followed the elimination of second-seeded Angelique Kerber of Germany in the second round on Tuesday, marking the first time the top two seeds had failed to reach the last eight at the tournament.
In other matches on Wednesday, fourth seed Caroline Wozniacki of Denmark booked her place in the quarter-finals with a 6-1 7-5 victory over Italy's Karin Knapp and Romania's Simona Halep battled past Spaniard Carla Suarez Navarro 3-6 6-4 6-1.
The New Haven Open is the final tune-up on the WTA Tour before the final grand slam event of the year, the U.S. Open, which starts on Aug. 26 in Flushing Meadows, New York.