She converted her third match point on a double-fault by Paszek, who has won only two out of six matches at her home event.
Mattek-Sand will play Carla Suarez Navarro of Spain, who defeated Magdalena Rybarikova 6-3, 6-4.
Julia Goerges beat 2007 champion Daniela Hantuchova 6-4, 7-6 (5). The fifth-seeded German closed out the victory with a forehand winner after trailing 5-3 in the second set. She will play Romina Oprandi, who defeated Alexandra Cadantu 6-2, 6-1.
Simona Halep of Romania led Lesia Tsurenko 3-0 in the opening set when the Ukrainian withdrew because of a right shoulder injury.
Also, Mallory Burdette of the United States beat Ksenia Pervak of Kazakhstan 6-3, 6-4. Pervak lost in qualifying but replaced 2009 champion Yanina Wickmayer after the Belgian withdrew shortly before the match because of fatigue.
Dominika Cibulkova, who reached last year's final, withdrew because of a shoulder injury.