Julen Lopetegui's side went into the match unbeaten in the league this season, but three draws in their last four matches had seen them fall four points behind Benfica.
Nevertheless, a comfortable triumph on Saturday has seen that gap slashed to just one point, at least until Benfica travel to Braga on Sunday.
Jackson Martinez doubled the lead a couple of minutes later, tapping in Yacine Brahimi's teasing delivery and Casemiro added their third just before the break, heading in off the crossbar.
Colombia international Martinez scored another tap-in with an hour gone from Cristian Tello's centre and Vincent Aboubakar netted his first Primeira Liga goal late on, finding the target following good work from Ricardo Quaresma.
The day's only other game saw Moreirense claim only their second win of the campaign, as they defeated rock-bottom Gil Vicente 2-0 thanks to second-half goals from Gerso Fernandes and Arsenio.
