American crowd favorite Sloane Stephens and four-time tournament champion Caroline Wozniacki were among the first-round winners Monday at the New Haven Open, a final U.S. Open tune-up.

The sixth-seeded Stephens got past Slovakian qualifier Anna Schmiedlova 6-4, 6-4 on the hardcourts at the Connecticut Tennis Center at Yale University.

Wozniacki, who titled here four straight years from 2008-11, had a tougher time in her opening match with Peng Shuai. The fourth-seeded former U.S. Open finalist dropped the first set but was leading the second when Peng retired due to dizziness to give Wozniacki the 2-6, 3-1 victory.

That match was delayed more than two hours by rain, with the extended stoppage forcing defending tournament champion Petra Kvitova's first-round bout to be postponed until Tuesday. The third-seeded Czech, who downed Russian Maria Kirilenko in last year's New Haven finale, will take on Urszula Radwanska.

Day-2 upsets came when nifty Spaniard Carla Suarez Navarro took out fifth- seeded Italian Roberta Vinci 7-5, 6-3 and Czech veteran Klara Zakopalova doused eighth-seeded Slovak Dominika Cibulkova 3-6, 6-3, 6-3.

Surging Romanian Simona Halep continued her recent winning ways with a 6-2, 6-1 pasting of wild card and fading Slovak Daniela Hantuchova. Halep has captured no less than three WTA titles over the last couple of months.

Also advancing were Russians Elena Vesnina, Ekaterina Makarova and Anastasia Pavlyuchenkova as well as a trio of qualifiers -- American Alison Riske, Puerto Rico's Monica Puig and Italian Karin Knapp.

Vesnina was the 2009 New Haven runner-up to Wozniacki.

Former French Open runner-up Sara Errani is the top seed this week and will face Makarova in the round of 16.

This week's winner will earn $117,000.