Vliegen, the first qualifier to reach the semis in the claycourt tournament in seven years, won 7-6 (7-1), 6-4 and will meet the No. 8 seed, Tommy Robredo of Barcelona. Robredo topped No. 5 Fernando Gonzalez of Chile 4-6, 7-5, 7-6 (7-2) in a lengthy match that was interrupted by rain.
"It's great to play at home and I got great crowd support today. I'm quite tired as it was very tough match but hopefully I can recover for the semifinals," Robredo said.
In the other semifinal, No. 13 Gaston Gaudio of Argentina meets Albert Montanes, a Spanish wild card.
Gaudio won over Brazilian Gustavo Kuerten, who retired in the third set with a groin injury. Montanes ousted seventh-seeded Agustin Calleri of Argentina 4-6, 7-6 (7-3), 6-3.