Kohlschreiber advanced when third-seeded Juan Martin del Potro pulled out due to inflammation in his right toe. Kohlschreiber will meet fifth-seeded Fernando Verdasco on Friday.
The eighth-seeded Monfils beat Radek Stepanek of the Czech Republic 6-4, 6-3 and will play second-seeded Fernando Gonzalez of Chile.
Lopez, who won the tournament in 2004, defeated Santiago Giraldo of Colombia 7-6 (5), 6-3. He will play Melzer, a 7-6 (4), 6-3 winner over Juan Carlos Ferrero.
Philipp Petzschner and Carlos Moya play in the fourth quarterfinal on Friday.
Monfils had two breaks to one in the first set. He broke twice more in the second after dominating with powerful forehand groundstrokes.
"My game was very solid today and I am glad that I stayed focused throughout the match,'' Monfils said.
Ferrero won just five points on Melzer's serve in the opening set and was broken twice in the second.
"I got the feeling today that I just couldn't do anything wrong,'' Melzer said. "I tried to be the first to take the initiative in every rally and I was hitting winners from all angles.''
Del Potro, who has won four ATP titles this season, broke a toe nail at the U.S. Open. The injury worsened during his win over Martin Fischer on Wednesday.
He was unable to play despite medical treatment early Thursday, organizers said.
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