Two-time champion and former world No. 1 Venus Williams was an easy first-round upset winner Monday at the U.S. Open.

The 33-year-old Venus leveled 12th-seeded surprise Wimbledon semifinalist Kirsten Flipkens, of Belgium, 6-1, 6-2 in 1 hour, 24 minutes at Arthur Ashe Stadium.

The currently 60th-ranked seven-time Grand Slam champion Venus captured the Open back-to-back in 2000 and 2001 and was the runner-up here in 1997 and 2002. Her second-round opponent will be China's Zheng Jie.

Meanwhile, third seed Agnieszka Radwanska and fifth-seeded Chinese star Li Na were also among the winners on Day 1.

The 2012 Wimbledon runner-up Radwanska, of Poland, routed Spaniard Silvia Soler-Espinosa 6-1, 6-2 at Ashe, while this year's Australian Open runner-up and the former French Open champion, Li, leveled Belarusian Olga Govortsova 6-2, 6-2 on the grounds at the USTA Billie Jean King National Tennis Center.

Up next for Radwanska will be Spaniard Maria-Teresa Torro-Flor, while Li will encounter Swede Sofia Arvidsson.

Also, 16th-seeded Wimbledon runner-up Sabine Lisicki, of Germany, handled Russian Vera Dushevina 6-2, 7-6 (7-3); 18th-seeded Spaniard Carla Suarez Navarro dismantled American Lauren Davis 6-0, 6-0; 19th-seeded Romanian Sorana Cirstea edged out Canadian Sharon Fichman 7-5, 5-7, 6-1; and 23rd-seeded American Jamie Hampton bested Spaniard Lara Arruabarrena 6-4, 6-2 for her first-ever U.S. Open match win.

Some other seeded winners on Monday were No. 25 Estonian Kaia Kanepi, No. 30 Brit Laura Robson and No. 32 Russian Anastasia Pavlyuchenkova. Kanepi overcame American Vania King in three sets, 4-6, 7-6 (7-4), 6-1.

A mild upset came when Austrian Patricia Mayr-Achleitner doused 29th-seeded Slovak Magdalena Rybarikova, 7-6 (7-2), 6-3.

Also advancing was American Coco Vandeweghe, who topped Serb Aleksandra Krunic 6-4, 7-6 (7-5).