Mathieu beat fifth-seeded fellow Frenchman Marc Gicquel, 6-3, 7-5.
Montanes eliminated fellow Spaniard Ramirez Hidalgo, 1-6, 7-5, 6-3.
Mathieu, Gicquel and Montanes reached the semifinals by eliminating the top three seeds in the tournament -- Dominik Hrbaty, Jose Acasuso and Julien Benneteau -- in Friday's quarterfinals.
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