VIENNA, Austria - Philipp Kohlschreiber of Germany, Feliciano Lopez of Spain, Gael Monfils of France and Juergen Melzer of Austria completed the quarter-final lineup at the BA Tennis Trophy event Thursday.

Kohlschreiber advanced when third-seeded Juan Martin del Potro of Argentina pulled out due to a toe injury. Kohlschreiber will meet fifth-seeded Fernando Verdasco of Spain on Friday.

Del Potro, who has won four ATP titles this season, broke a toe nail at last month's U.S. Open and the injury worsened during his first-round win over Martin Fischer of Austria on Wednesday.

He was unable to play despite medical treatment early Thursday, organizers said.

The eighth-seeded Monfils beat Radek Stepanek of the Czech Republic 6-4, 6-3 and will play second-seeded Fernando Gonzalez of Chile.

"My game was very solid today and I am glad that I stayed focused throughout the match," Monfils said.

Monfils had two breaks to one in the first set. He broke twice more in the second after dominating with powerful forehand groundstrokes.

Lopez, who won the tournament in 2004, defeated Santiago Giraldo of Colombia 7-6 (5), 6-3 to set up a quarter-final match against Melzer.

Melzer defeated former top-ranked Juan Carlos Ferrero of Spain 7-6 (4), 6-3.

"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."

Ferrero won just five points on the Austrian's serve in the opening set and was broken twice in the second.

Philipp Petzschner of Germany and Carlos Moya of Spain play each other in the fourth quarter-final match Friday.