By Chick Young
BBC Scotland at Celtic Park
Celtic survived an early scare against Championship side St Mirren to reach the Scottish Cup semi-finals.
The visitors took a shock lead on 13 minutes when John Sutton nodded down for Harry Davis to tap home.
Two Celtic goals in as many second-half minutes from Mikael Lustig and Scott Sinclair turned the match on its head and the home side never looked back.
Moussa Dembele and substitute Leigh Griffiths struck as the 36-times winners eased into the last four.
In essence this was a meeting of the form teams of Scottish football and Celtic paid Saints the respect of naming an unchanged side. For all that, the Paisley club were being offered at 40-1 to win the tie.
In a match preceded by a minute's silence in honour of Lisbon Lion Tommy Gemmell who died this week, emotion was thick in the air.
Incredibly, though, it was St Mirrren who took the lead. It was a cleverly worked free-kick down the left with Mallan set up Smith with a disguised pass to the byline.
His cross was nodded back into the heart of the six yard box by Sutton and Davis was there to turn the ball home.
As the match progressed it became clear Saints were equipped to take this test of Celtic further. Outwith the Champions' League they were giving Brendan Rogers' side their biggest test of the season at Parkhead.
In the wake of the goal they sat in with a very disciplined 5-3-2 formation and survived some close calls, the pick of which was a Scott Brown cut-back of which the usually razor sharp Dembele made a total hash.
But they held on until the break and St Mirren were now 45 minutes from one of the great Scottish Cup upsets.
Brendan Rogers replaced the ineffective Gary Mackay-Steven with Patrick Roberts for the start of the second half.
Seven minutes into the second period Celtic had a remarkable escape. Stevie Mallan drove in a low free-kick to the near post which Dembele managed to divert against his own crossbar. It proved to be the turning point of the match.
Wth 58 minutes on the clock Celtic were level. Roberts' delivery into the box from a free-kick out wide was good and Lustig headed home to give his team a grip on the tie again.
Not much more than a minute later they were ahead with a quite audacious goal from Sinclair. The Saints defence looked to have the situation under control but somehow the striker opened his foot and curled an outrageous effort into the top corner.
It was over as a contest now. Celtic made it 3-1 with an electric break with Sinclair at its heart. He powered forward, released Roberts, took the return and laid it on a plate for Dembele to tap home.
Saints were suffering in a way they didn't deserve. Griffiths, who had a huge influence on the game after coming off the bench, added a fourth with a fine left foot shot from just inside the box and the Paisley side were out on the floor.
Celtic march on to the semi-finals. St Mirren's thoughts turn back to Championship survival.