MADRID - Top-ranked Serena Williams was knocked out of the Madrid Masters on Wednesday, falling 4-6, 6-2, 6-3 to Nadia Petrova of Russia in the third round.

Petrova, who has won 19 doubles titles, had 10 aces and frustrated Williams into making 41 errors.

Williams had looked out of form in a marathon match against Vera Dushevina on Tuesday, which lasted 3 hours, 26 minutes. It was the longest match of her career.