Third seed Juan Martin Del Potro of Argentina withdrew from the Vienna [Images] Open on Thursday because of a niggling toe injury.

The world number nine, who won his first four ATP titles in a brilliant consecutive run in July and August, has been struggling with a broken nail on his right foot since the US Open [Images].

After suffering further inflammation overnight the 20-year-old Del Potro pulled out of his second-round match with Philipp Kohlschreiber, allowing the German through to a quarter-final meeting with Spanish fifth seed Fernando Verdasco.

French eighth seed Gael Monfils also advanced with a 6-4, 6-3 triumph over Czech Radek Stepanek.

In the top half of the draw unseeded Spaniard Feliciano Lopez defeated Colombian Santiago Giraldo 7-6, 6-3.

Lopez, whose only previous ATP singles title came in Vienna four years ago, next meets local favourite Juergen Melzer.

The unseeded Melzer, the last Austrian left in the competition, saw off former world number one Juan Carlos Ferrero of Spain 7-6, 6-3.

• Complete tennis coverage
• All you want to know about Sania Mirza