Melzer, the crowd favorite since he's from Austria, defeated Agustin Calleri, 6-4, 7-5. He took the second set with a strong series of service games during which he didn't face a break point and won 69 percent of the points.
Youzhny had an easier time with a 6-2, 6-4 win over Stefan Koubek behind four aces and four service breaks. Youzhny plays Melzer on Thursday.
Third-seeded Victor Hanescu qualified for the quarterfinals with a 6-1, 7-6 (7-2) win over Marcel Granollers and will go against Guillermo Garcia-Lopez on Thursday.
Wild-card entrant Daniel Koellerer took out Diego Junqueira, 6-1, 6-2, and will meet Julien Benneteau, a 6-1, 6-7 (4-7), 7-6 (9-7) winner Wednesday over Nicolas Massu, in the quarters.
Oscar Hernandez beat Andreas Beck, 7-5, 6-4, and Juan Ignacio Chela topped Ruben Ramirez, 6-3, 6-2, with the winners paired against each other in Thursday's quarterfinals.