The two will meet in the quarterfinals after Almagro defeated fellow Spaniard Marcel Granollers 6-3, 6-4 on the outdoor red clay. Monaco outlasted Frenchman Jeremy Chardy in a long second-set tiebreaker to win 6-0, 7-6 (11).
The showers held off at the Real Club de Tenis until former No. 1 Juan Carlos Ferrero was honored after Almagro's match. Ferrero retired last summer at age 32.
Tournament director Albert Costa said the forecast called for more rain on Friday, and some competitors may have to play back-to-back matches.
"I feel bad because we may have games on courts that are not ideal," Costa said. "But the players are physically prepared for it and know that it is a possibility."