LONDON — Andy Roddick reached the quarterfinals at Queen's Club on Thursday when fellow American Mardy Fish retired with a left ankle injury after losing the opening set 7-6 (5).

Roddick, the defending champion, struggled to gain the upper hand and faced two break points at 2-2 before holding. Fish, who in March defeated Roger Federer to reach the final at Indian Wells, Calif., held off three break points in the next game, hitting two aces and a forehand winner before holding with his seventh ace.

A single mini-break for 2-0 in the tiebreaker was enough for Roddick to win the set.

Fish, who had fallen on the slippery surface, received treatment on his ankle and decided to stop playing.

Ivo Karlovic, who lost to Roddick in the 2005 final, upset 10th-seeded Fernando Gonzalez 6-3, 7-5.