Lucie Safarova joined the US Open seeds casualty list on Tuesday when the Czech left-hander became the fourth top-10 player to lose in the first round.
The sixth-seeded Czech slipped to a 6-4 6-1 defeat to Ukraine’s Lesia Tsurenko.
Safarova, the French Open runner-up, joins top-10 seeds Ana Ivanovic, Karolina Pliskova and Carla Suarez Navarro in failing to get out of the first round.
With world No.3 Maria Sharapova having pulled out of the tournament through injury, Safarova’s defeat also served to ease world No.1 Serena Williams’s path to a calender Grand Slam.
Williams reached the second round on Monday when Vitalia Diatchenko retired with a foot injury after just 30 minutes.
World No.2 Simona Halep avoided the upsets when she reached the second round after he New Zealand opponent Marina Erakovic retired with a knee injury.
Halep was leading 6-2 3-0 when her 99th-ranked rival called it quits, having received a medical timeout for treatment on her right knee at the end of the first set.
Another early winner Tuesday was German 11th seed Angelique Kerber, a 2011 semi-finalist, who saw off Romania’s Alexandra Dulgheru 6-3 6-1.