Montanes will be appearing in his 10th final but first since July 2010, when he won his fifth career title in Stuttgart, Germany. He and Haase have never played.
Montanes broke the 23rd-ranked Chela's serve three times to take the opening set before rain halted play for almost an hour. The Spaniard went a break up in the second set but failed to close the match while serving at 5-4. He did so at 6-5 when Chela sent a forehand into the net.
The 53rd-ranked Haase was playing in his second semifinal, the other coming at the Dutch Open in 2007. He won 16 of the first 20 points against Souza and dropped just one point on serve in the opening set.
Souza failed to serve out the second set at 5-4 before clinching it in the tiebreaker. He lost serve again in the opening game of the deciding set.
The 110th-ranked Brazilian was trying to become the first qualifier this season to reach an ATP final.