It was Ferrer's 13th consecutive victory over his Spanish Davis Cup teammate.
Almagro served for the match in the 10th game of the third set and the 10th and 12th games of the fourth set, but was broken each time by Ferrer.
The fourth-seeded Ferrer will play either top-ranked Novak Djokovic, the defending champion, or Tomas Berdych in the semifinals.
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