Melzer slipped past Mischa Zverev 7-5, 7-5. He had a shaky serve, landing only 58 percent of his first serve, but he managed to save four of six of Zverev's break-point changes and took the match because of four breaks of his own.
He moves into Friday's quarterfinals against fifth-seeded Albert Montanes, who claimed a 7-5, 3-6, 6-2 win over Juan Ignacio Chela. Montanes piled up eight service breaks over 14 receiving games.
Simon Gruel ousted seventh-seeded Gilles Simon 6-4, 7-5, dropping serve only once and winning 70 percent of the points on serve.
He'll have to wait to learn whom he's playing in the quarterfinals, however, as a match between No. 4-seeded Juan Carlos Ferrero and Horacio Zeballos was suspended because of darkness. Zeballos will have a 7-6 (7-1), 6-6 lead when play resumes Thursday.
The Ferrero-Zeballos winner will draw Gruel in Friday's schedule.
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