Morton stunned Premiership Motherwell to become the first side through to the Scottish Cup quarter-finals.
The second-tier side from Greenock led when Robbie Muirhead's corner went in off Motherwell's Harry Paton.
George Oakley, who had earlier struck the woodwork, doubled the hosts' advantage with a firm low drive.
The visitors reduced their deficit when Jack Vale's shot deflected into the net off Darragh O'Connor, but they could not avoid a first defeat of 2024.
The draw for the last eight will take place on Sunday on BBC One Scotland during Sportscene's coverage of Airdrieonians v Hearts, with the programme starting at 16:30 GMT.
And 1922 winners Morton, who are unbeaten in 13 games, will aim to reach the semi-finals of the competition for the first time since 1981.
Motherwell goalkeeper Liam Kelly avoided early embarrassment when he spilled a free-kick, the Scotland international recovering before anyone could capitalise.
Dougie Imrie's hosts had another near thing when Oakley latched on to O'Connor's header and lifted a shot on to the crossbar.
The visitors simply could not get into a rhythm and fell behind from Muirhead's set-piece. The fierce delivery from the right was palmed by Kelly on to Paton and back over the line.
The second period followed much in the vein of the first. Oakley pounced on a loose ball, bore down on goal and struck through the legs of Calum Butcher to beat Kelly.
A triple change from Motherwell that gave Jili Buyabu his debut and also introduced Adam Devine and Vale was followed by Andy Halliday and fit-again Jon Obika appearing from the bench as manager Stuart Kettlewell tried to engineer a comeback.
And Motherwell rallied. Theo Bair threatened with a header that Ryan Mullen claimed comfortably then Vale curled an effort wide shortly before he collected Devine's pass and scored via O'Connor's intervention.
The final few minutes were tense but Morton stood firm.
Player of the match - George Oakley
What they said
Morton manager Dougie Imrie: "I'm absolutely delighted. This is a massive victory for the club. I'm really happy for my players. We managed to get the second goal at the right time.
"The back four handled [Blair Spittal and Theo Bair] very well. We tried to pack the midfield. To a man, my players were outstanding and I'm proud of them. If we can get a home tie and we can get this kind of support, we'll take anybody."