Motherwell eased to victory over Morton to make sure there was no Scottish Cup shock at Fir Park.
Second Division leaders Morton, who knocked out Kilmarnock in the previous round, went close early on through Chris Templeman.
But Brian Kerr fired the Scottish Premier League side ahead from the edge of the box after just 10 minutes.
Morton again had chances, but Scott McDonald drove the ball into the far corner to end the visitors' hopes.
David Clarkson and Jim Paterson had come back from suspension for Motherwell, with Steven McGarry dropping to the bench.
On-loan Auxerre midfielder Krisztian Vadocz was beside him following the sale of Richie Foran to Southend.
Morton brought in Kevin Finlayson for Jamie Stevenson.
Kick-off was delayed for 10 minutes as long queues of fans waited outside Fir Park.
Well shaded the early possession but survived a scare when Templeman was inches away from Finlayson's dangerous cross.
But the hosts took the lead after 10 minutes when a Darren Smith shot was blocked and Kerr thumped the ball home from 18 yards.
Morton tried to battle back and Scott McLaughlin had a powerful shot beaten away by Graeme Smith.
The Greenock side had a great chance to level when Finlayson played through Paul McGowan, but the striker's lob cleared the post.
But Motherwell went two ahead after 34 minutes when McDonald won the ball in the penalty area and cut inside to fire the ball under Paul Mathers.
The Australian almost added to the scoreline with a lob that was just diverted by Mathers.
Morton came close three minutes after the restart when Templeman headed David McGregor's inswinging free-kick towards the back-post, where Peter Weatherson just failed to turn it in.
Clarkson had a powerful shot from a tight angle held by Mathers at the other end after a long ball by Martyn Corrigan.
Stephen Craigan made a superb goalmouth interception from McGowan as Morton continued to probe.
But Morton ran out of ideas as the game entered the final quarter and Well finally began to knock the ball around for spells without threatening Mathers.
Motherwell: Graeme Smith, Corrigan, Craigan, Reynolds, Murphy, Darren Lee Smith (McGarry 68), Kerr, Fitzpatrick, Paterson, McDonald, Clarkson (Keegan 82).
Subs Not Used: Meldrum, Quinn, Vadocz.
Booked: McDonald.
Goals: Kerr 10, McDonald 34.
Morton: Mathers, Weatherson, Harding, Greacen, Macgregor, Finlayson, McGowan, McLaughlin, McAlistair, Templeman (Keenan 82), Millar (Stevenson 58).
Subs Not Used: McGurn, Walker, Russell.
Att: 9,394
Ref: J Underhill
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