Serena among first-round winners in Madrid

Defending champion Serena Williams and fourth-seeded Agnieszka Radwanska were among Sunday's first-round winners at the Madrid Open.

Williams recovered from a slow start to beat qualifier Yulia Putintseva of Kazakhstan by a 7-6 (7-5), 6-1 margin. The American defeated Victoria Azarenka in last year's Madrid finale.

Radwanska of Poland cruised past Bulgaria's Tsvetana Pironkova, 6-2, 6-4, and American Madison Keys took down fifth-seeded Li Na of China by a 6-3, 6-2 margin.

There was two other upsets on Sunday. Kazakh Yaroslava Shvedova eased past Caroline Wozniacki, the No. 10 seed, by a 6-2, 6-4 margin and American Varvara Lepchenko outlasted 12th-seeded Italian Roberta Vinci 6-1, 1-6, 6-2.

Elsewhere, Venus Williams was forced to withdraw from the tournament due to a back injury. Stefanie Voegele entered the draw as a lucky loser before falling to Anabel Medina Garrigues by a 6-3, 6-3 margin.

Sixth-seeded Angelique Kerber beat Su-Wei Hsieh 3-6, 6-3, 6-2 and No. 8 seed Petra Kvitova outlasted Yanina Wickmayer 4-6, 7-5, 6-4.

Also posting first-round victories on Sunday were No. 11 seed Nadia Petrova, No. 13 seed Maria Kirilenko, No. 14 seed Marion Bartoli, No. 15 seed Dominika Cibulkova, No. 16 seed Ana Ivanovic, Chanelle Scheepers, Svetlana Kuznetsova, Kaia Kanepi, Sorana Cirstea, Ekaterina Makarova and Maria-Teresa Torro-Flor.