The Frenchman returned to the court for the first time since the Australian Open in January after recovering from a knee problem and duly saw off Spaniard Montanes 6-4 5-7 6-4 in Casablanca.
The third seed fired down 13 aces and took three of the seven break points that came his way to reach the last eight of a tournament which top seed Kevin Anderson and seventh seed Robin Haase crashed out of on Wednesday.
There was no sign of rustiness from Paire as he broke to take a 2-1 lead and saved a break point in the following game before going on to serve out the set.
The writing appeared to be on the wall for Montanes when Paire broke in the second set to take a 3-2 lead, but the latter was unable to serve out the match as his opponent hit back to set up a decider.
Paire was not to be denied, though, as he grabbed one further break in the third set to secure a winning return.
Another Spaniard, Guillermo Garcia-Lopez, stands in the way of Paire and a place in the semi-finals after he beat Carlos Berlocq 6-2 7-5.
Garcia-Lopez, the eighth seed, has now reached the last eight in Casablanca in three of the last four years and never looked back after winning the first four games against his Argentine opponent on Thursday.
Fifth seed Joao Sousa crashed out as qualifier Roberto Carballes Baena battled to a 6-7 (5-7) 7-6 (7-5) 6-2 victory in three hours and 13 minutes.
Carballes Baena will now face Russian Andrey Kuznetsov, who was leading Jiri Vesely 6-2 3-1 when his opponent cut short his tournament due to illness.
Marcel Granollers won an all-Spanish encounter against Albert Ramos, while Pablo Carreno Busta was yet another Spaniard to progress after he beat Aleksandr Nedovyesov.