Both teams have had chances to win it in regular time, but another 30 minutes it is.

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St Johnstone squander a massive chance to win it.

Jason Holt is in acres of space and has time to pick his spot, but he slides his shot inches wide.

An ultimately comfortable afternoon for Thistle as they breeze past Ayr United into the quarter-final draw.

Daniel O'Reilly's looping header and Logan Chalmers' fizzing strike before the interval were enough to see off a Ayr side.

Ayr grew into the game as the second half went on, but failed to trouble Thistle's defence.

Harry Milne causes havoc in the St Mirren box and Elton Kabangu misses a big chance before Lawrence Shankland's effort is blocked.

An offside flag brings the episode to an end, but Hearts may live to regret that one.

Four minutes added on in Perth.

This is surely heading for extra-time.

Well, a few were expecting Aberdeen to have a rough ride this afternoon after their sluggish start to the season.

They were rarely troubled by Morton, bar a brief spell at the start of the second half.

Goals from Sivert Heltne Nilsen and Kusini Yengi before the break were followed by Leighton Clarkson's lovely dink with 10 minutes left.

It wasn't perfect from the Dons, but a solid cup display and they are in the hat for the quarter-final draw.

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Brian McLauchlin
BBC Scotland at Smisa Stadium, Paisley

From nowhere Hearts now look the dominant side and it's Stephen Robinson who now appears the more frustrated on the touchline.

Cracking cup tie!

Now St Mirren ring the changes as Tunmise Sobowale, Roland Idowu and Evan Mooney replace Jayden Richardson, Mikael Mandron and Keanu Baccus.

Cheick Diabate nearly finds the back of his own net after another threatening ball in from Motherwell.

It goes over the bar though and it remains 0-0.

Motherwell continue to pile forward and win a corner after a driving run from Tom Sparrow down the right.

St Johnstone clear it easily, before Josh McPake's shot from the halfway line goes a good 20 yards wide.

Seven minutes left of the 90 in Perth.

Alan Forrest comes on for Craig Halkett in Hearts' latest change. That one was in the post after he seemed to pull a muscle.

Moments after Lawrence Shankland forces a Hearts corner with an effort from the right, team-mate Craig Halkett goes down clutching his hamstring.

He's had a tough day at the office so far with that first-half head injury. He's continuing for now but looks like he might need to come off.

Motherwell continue to frustrate.

They have a five on three in attack, which ends with an Ibrahim Said shot being deflected behind for a corner.

Not a great deal happening at Cappielow.

Aberdeen have not really been at it in the second half but haven't really needed to be as Morton have struggled to put the Premiership side under any pressure.

Claudio Braga forces a save from St Mirren goalkeeper Shamal George, who holds.

Better from Hearts, they're not quite knocking at the door but they're on their way up the footpath.

George Oakley twists around the ball in the area, but his tame effort from inside the six-yard box is dealt with.

Ayr have been toothless all afternoon.

Almost the classic sucker-punch.

Having dominated the game, Motherwell nearly go behind.

Uche Ikpeazu flicks a long ball on for Makenzie Kirk to chase and the Saints sub is only denied by a good save from visiting goalkeeper Calum Ward.

An hour in, Hearts are starting to exert themselves in the final third but cannot find that telling pass or shot.

Lawrence Shankland batters one from the edge of the box, but it's charged down then a low ball slowly traverses the St Mirren area only to be tidied up by a defender.