Celtic narrowly beat Second Division Arbroath to earn a Scottish Cup fifth-round tie at Raith Rovers.
Neil Lennon's men dominated early on and deservedly went ahead thanks to a fabulous Adam Matthews drive.
But Arbroath refused to buckle and made a real contest of it, Steven Doris seeing a goal disallowed after the break for a foul earlier in the move.
The match ended with Celtic trying to run down the clock, which was testament to the challenge posed by the hosts.
They could not fashion an equaliser but emerge from the tie - over the two games - with enormous credit.
Celtic have had few tougher tests over the course of the season but started well enough, controlling the early stages, playing an unfamiliar 3-5-2 formation.
Georgios Samaras sent a header straight at Scott Morrison in the Arbroath goal on five minutes and, shortly afterwards, Lassad had the ball in the net, only for the assistant to flag for offside.
Ironically, though, the opening goal came just as Arbroath had looked like threatening the Celtic goal.
Paul Sheerin had a shot charged down and Celtic burst upfield, Matthews powering through before unleashing a splendid shot that flew past Morrison and into the top corner.
The breakthrough came on 18 minutes and though Celtic were comfortable, it was another 15 minutes before they troubled Morrison again.
Charlie Mulgrew arrowed a delightful diagonal pass to Samaras, who in turn cushioned it for Lassad and the Tunisian should have done much better than shoot tamely at the keeper from eight yards.
A free-kick early in the second half caused a degree of discomfort in the Celtic penalty area, Stuart Malcolm lunging at the ball to try to poke it over the line, but instead earning himself a yellow card for catching Forster with his raised boot.
Arbroath thought they had grabbed the equaliser with just over an hour on the clock as another Sheerin delivery caused consternation, with Forster failing to gather and the ball being thumped into the net by Doris, but referee Craig Charleston ruled the effort out for an infringement.
Though Brown and Beram Kayal flashed shots just wide, manager Lennon was not content with his side's display and replaced Emilio Izaguirre and the ineffective Lassad with Victor Wanyama and Gary Hooper.
The finely-balanced nature of the tie was underlined by a passage of play during which Kayal brought out a flying save from Morrison before Arbroath substitute Connor Birse broke free only to see Forster divert his shot wide with his trailing foot.
That was to be Arbroath's final chance, as it turned out, but Celtic could not relax until the final whistle, which they greeted with relief as much as delight.