World No.1 Rafael Nadal breezed into the quarter-finals of the ATP Rio Open on Thursday but admitted that he was still feeling the effects of a back injury.

Nadal, in his first event since losing last month’s Australian Open final, needed only 74 minutes to rout Spanish compatriot Albert Montanes 6-1 6-2 and book a last-eight clash with Portugal’s 48th-ranked Joao Sousa, who eliminated 87th-rated Spaniard Albert Ramos 7-6 (8-6) 2-6 6-3.

Nadal wore elastic tape across his lower back during the match and said the injury he suffered in losing the championship decider in Melbourne to Stanislas Wawrinka still nagged him.

“When you get injured in the final of a grand slam and stop for two and a half weeks, it is not easy to come back,” Nadal said.

“The back is still bothering me a little bit but I am happy with my decision to be here.”

Nadal broke Montanes three times in each set and won the final four games of the match as he improved to 3-0 all time against his countryman, having yet to lose more than three games in any set against him.

In career matches against Spaniards, Nadal is 101-15.

The sore back forced the 13-time grand slam champion to skip Buenos Aires last week, leaving his return for the $US1.3 million ($A1.45 million) tournament in Rio, where he starts a claycourt season that he hopes will culminate in a ninth career French Open crown.

Nadal, who reached a career-high 14 finals in 2013, seeks a berth in his third consecutive final, having won at Doha in the first week of the year.