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.
Arbroath thought they had levelled in the second half
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.
Full Time The game is over, as the referee blows his whistle.
Colin Hamilton fouled by Georgios Samaras, the ref awards a free kick. Scott Morrison takes the direct free kick.
Georgios Samaras restarts play with the free kick.
90:00 +0:50 Booking
Booking Scott Robertson booked for unsporting behaviour.
Free kick awarded for an unfair challenge on Connor Birse by Dylan McGeouch. Scott Morrison restarts play with the free kick. Scott Robertson gives away a free kick for an unfair challenge on Victor Wanyama.
Substitution Dylan McGeouch is brought on as a substitute for Adam Matthews.
Free kick taken by Paul Currie.
Booking Georgios Samaras booked.
Georgios Samaras concedes a free kick for a foul on Paul Currie.
Scott Robertson fouled by Gary Hooper, the ref awards a free kick. Colin Hamilton produces a shot on goal direct from the free kick.
Paul Currie takes a outswinging corner from the right by-line played to the near post, clearance by Mikael Lustig.
Substitution Connor Birse replaces Brian Kerr.
Substitution Paul Sheerin leaves the field to be replaced by Derek Holmes.
Scott Robertson fouled by Mikael Lustig, the ref awards a free kick. Steven Doris fires a strike on goal direct from the free kick.
Substitution Daniel Rennie joins the action as a substitute, replacing Lee Sibanda.
Efe Ambrose challenges Steven Doris unfairly and gives away a free kick. Paul Currie takes the direct free kick.
Steven Doris fouled by Victor Wanyama, the ref awards a free kick. Free kick taken by Paul Sheerin.
Substitution Emilio Izaguirre goes off and Victor Wanyama comes on.
Substitution Gary Hooper replaces Lassad Nouioui.
Inswinging corner taken by Paul Sheerin from the right by-line played to the near post, Efe Ambrose manages to make a clearance. Corner taken by Paul Sheerin from the left by-line to the near post, save made by Fraser Forster. Alexander Keddie gives away a free kick for an unfair challenge on Fraser Forster. Fraser Forster takes the free kick.
Georgios Samaras is ruled offside. Scott Morrison takes the direct free kick.
Steven Doris gives away a free kick for an unfair challenge on Kelvin Wilson. Fraser Forster takes the direct free kick.
Charlie Mulgrew fouled by Steven Doris, the ref awards a free kick. Charlie Mulgrew restarts play with the free kick.
Fraser Forster takes the free kick.
Booking Stuart Malcolm is cautioned.
Paul Sheerin takes the free kick. Free kick awarded for an unfair challenge on Fraser Forster by Stuart Malcolm.
Booking Charlie Mulgrew receives a caution for unsporting behaviour.
Charlie Mulgrew concedes a free kick for a foul on Paul Currie.
Inswinging corner taken from the right by-line by Charlie Mulgrew, Stuart Malcolm makes a clearance.
The second half has started.
45:00 +0:28 Half time
Half Time The first half comes to an end.
Charlie Mulgrew takes the inswinging corner, Scott Morrison makes a save.
Booking Brian Kerr is cautioned by the ref for dissent.
Free kick awarded for an unfair challenge on Charlie Mulgrew by Lee Sibanda. Free kick taken by Charlie Mulgrew.
Scott Brown gives away a free kick for an unfair challenge on Paul Currie. Direct free kick taken by Brian Kerr.
Unfair challenge on Steven Doris by Kelvin Wilson results in a free kick. Free kick taken by Paul Sheerin.
Shot from long range by Charlie Mulgrew misses to the right of the target.
Efe Ambrose produces a right-footed shot from a long way out which goes wide right of the goal.
Corner taken left-footed by Paul Sheerin from the right by-line, clearance by Efe Ambrose.
Foul by Lee Sibanda on Beram Kayal, free kick awarded. Charlie Mulgrew restarts play with the free kick.
The ball is sent over by Adam Matthews, save by Scott Morrison.
Corner taken left-footed by Paul Sheerin.
The official flags Georgios Samaras offside. Free kick taken by Scott Morrison.
The assist for the goal came from Mikael Lustig.
17:21 Goal scored
Goal! - Adam Matthews - Arbroath 0 - 1 Celtic A goal is scored by Adam Matthews from the edge of the penalty area to the top left corner of the goal. Arbroath 0-1 Celtic.
Kelvin Wilson concedes a free kick for a foul on Steven Doris. Paul Sheerin takes the free kick.
Paul Currie concedes a free kick for a foul on Beram Kayal. Beram Kayal restarts play with the free kick.
Scott Robertson gives away a free kick for an unfair challenge on Emilio Izaguirre. Charlie Mulgrew restarts play with the free kick. Georgios Samaras handles the ball and concedes a free kick. Scott Morrison takes the direct free kick.
The official flags Lassad Nouioui offside. Scott Morrison takes the free kick.
Inswinging corner taken from the right by-line by Charlie Mulgrew, Alexander Keddie makes a clearance. Charlie Mulgrew takes the inswinging corner, Charlie Mulgrew takes a shot.
Georgios Samaras takes a shot. Save by Scott Morrison.
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