Spain's Rafael Nadal hits a return to Colombia's Robert Farah during their Barcelona Open tennis match April 26, 2012.[Photo/Agencies]
"The truth is that there are few easy matches on the tour," world number two Nadal, chasing a seventh Conde de Godo trophy in eight years, said in an interview with Spanish television.
"Even though at the beginning I pulled away very quickly the second set was significantly more difficult," added the 25-year-old Mallorca native.
"He serves fast and very well and obviously that makes it tough but I am in the quarter-finals with positive feelings and let's see what happens tomorrow, which will be more difficult."
Nadal, fresh from winning a record eighth successive Monte Carlo Masters crown, was joined in the last eight by second seed Andy Murray after the Briton thumped Farah's unseeded compatriot Santiago Giraldo 6-1 6-2.
Murray has a poor record at the event in the Catalan capital and has failed to get past the second round in his three previous appearances.
The world number four will play Milos Raonic for a place in Saturday's semis after the 11th-seeded Canadian beat sixth seed Nicolas Almagro of Spain 6-3 6-3.