KEY BISCAYNE, Fla. (CBS/AP) – Maria Sharapova and Serena Williams both pulled off victories as action continued Monday at the Sony Ericsson Open on Key Biscayne.

Sharapova overcame 11 double-faults Monday to beat Ekaterina Makarova 6-4, 7-6 (3) and become the first quarterfinalist at the Sony Ericsson Open.

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Sharapova, seeded No. 2, played for more than two hours in 85-degree sunshine. She won despite committing 52 unforced errors and losing her serve four times.

Sharapova seeks her first Key Biscayne title after losing in the final three times.

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Serena won her match against the sixth-seeded Samantha Stosur.

Williams advanced to the quarterfinals with a straight set victory over Stosur, 7-5, 6-3.

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On the men’s side, No. 8-seeded Mardy Fish assured he’ll remain the top-ranked American by beating No. 28 Kevin Anderson in the third round, 6-4, 6-3.