Serena Williams overcame a sluggish start to beat Caroline Wozniacki 1-6, 6-3, 6-2 and reach the quarter-finals of the Madrid Open.

Williams hit the net on three straight points in her opening service game to be broken and then didn't put up much resistance in the rest of the first set. But she broke Wozniacki to love in the fifth game of the second and then ripped a forehand return winner for another break that put her ahead 4-3.

Wozniacki had no reply as the Dane hit a shot long to go down 2-0 in the decider, and Williams cruised to victory in a matchup of former top-ranked players.

Williams improved her record to 10-0 on clay this season with her third win in the Spanish capital.

Williams will play second-seeded Maria Sharapova after her Czech opponent Lucie Safarova withdrew before their match due to illness.

Fourth-seeded Agnieszka Radwanska of Poland knocked off RobertaVinci 7-6 (1), 6-4 and will play Varvara Lepchenko of the United States after she beat local favourite Anabel Medina Garrigues 6-1, 6-7 (3), 6-3.

Czech player Lucie Hradecka defeated Russia's Ekaterina Makarova 6-2, 7-6 (5) to advance to a meeting with fifth-seeded Sam Stosur.