Romanian second-seed Simona Halep eased into the second round of the US Open, advancing 6-2 3-0 after New Zealand's Marina Erakovic retired with a knee injury.

The in-form Halep, who reached the finals at US Open tune-up events in Toronto and Cincinnati, barely broke a sweat on a sunny Arthur Ashe Stadium court, needing a mere 47 minutes to see off the 99th ranked New Zealander.

Top-seed Serena Williams, bidding to complete a calendar sweep of the four grand slams, advanced on Monday in similar fashion as she was up 6-0 2-0 when Russia's Vitalia Diatchenko was forced to retire hurt.

While the top two women's seeds steered clear of trouble, upsets continued up and down the women's draw for a second consecutive day as sixth seed and French Open finalist Lucie Safarova fell 6-4 6-1 to Ukrainian Lesia Tsurenko.

With the loss, Safarova joins a number of first round casualties that already includes Serbian seventh seed Ana Ivanovic, eighth seeded Czech Karolina Pliskova and 10th-seed Carla Suarez Navarro of Spain.

"I wasn't surprised [by the upsets], it is normal," said 23-year-old Halep.

"Everyone is fighting like crazy because it's the last grand slam [of the year]. When you play the grand slam you have just to give everything you have.

Former world number one Wozniacki saw off US national champion Jamie Loeb [AP]

"I had this year already two grand slams where I lost in first and the second round, so like I say always, everything can happen."

Also sprinting to victory was fourth seed Caroline Wozniacki, who dismissed U.S. national college champion Jamie Loeb 6-2 6-0, and fifth-seeded, two-times Wimbledon winner Petra Kvitova beat Germany's Laura Siegemund 6-1 6-1.

Results (selected):

5-Petra Kvitova (Czech Republic) beat Laura Siegemund (Germany) 6-1 6-1
Yanina Wickmayer (Belgium) beat Francesca Schiavone (Italy) 6-3 6-1
18-Andrea Petkovic (Germany) beat Caroline Garcia (France) 3-6 6-4 7-5
32-Anna Karolina Schmiedlova (Slovakia) beat Julia Goerges (Germany) 6-3 6-4
9-Garbine Muguruza Blanco (Spain) beat Carina Witthoeft (Germany) 6-2 6-4
20-Victoria Azarenka (Belarus) beat Lucie Hradecka (Czech Republic) 6-1 6-2
Elena Vesnina (Russia) beat Laura Robson (Britain) 3-6 6-3 7-5
4-Caroline Wozniacki (Denmark) beat Jamie Loeb (U.S.) 6-2 6-0
Karin Knapp (Italy) beat Ajla Tomljanovic (Croatia) 6-7(1) 6-2 6-4
Olga Govortsova (Belarus) beat 28-Irina Begu (Romania) 6-1 0-6 7-6(3)
11-Angelique Kerber (Germany) beat Alexandra Dulgheru (Romania) 6-3 6-1
Lesia Tsurenko (Ukraine) beat 6-Lucie Safarova (Czech Republic) 6-4 6-1
2-Simona Halep (Romania) beat Marina Erakovic (New Zealand) 6-2 3-0 (Erakovic retired)
Shelby Rogers (U.S.) beat Sachia Vickery (U.S.) 6-2 6-2
Evgeniya Rodina (Russia) beat Tereza Mrdeza (Croatia) 6-2 6-2
16-Sara Errani (Italy) beat Mayo Hibi (Japan) 6-0 6-1
24-Sabine Lisicki (Germany) beat Aliaksandra Sasnovich (Belarus) 6-1 6-4

Source: Reuters