Williams is seeking her 47th career title, already holding the record among active players -- her older sister Venus is second with 44.

CNN: No. 1 Azarenka ruled out by foot problem

Serena's older sister Venus will play her second tournament since losing to Radwanska in the Miami quarterfinals in late March, having lost in the second round in Madrid.

CNN: Serena Williams thrashes No. 1 Azarenka in Madrid final

In Monday's other second-round clashes, Serena's fourth-seeded older sister Venus beat Russia's Vera Zvonareva 7-5 6-3.

CNN: Serena survives epic in Madrid opener