A dreadful tie came alive in the final minutes of extra time when Stephen O'Donnell scored a 35-yarder for Motherwell and Markus Fjortojft headed a last-gasp equaliser.

Now for the shootout...

Image source, BBC Sport

What a twist in the tale!

Fjortoft rises highest at a Morton corner to thump a header into the net and force penalties.

Image source, BBC Sport

Oooftt!

Out of nowhere Motherwell are ahead with a quite incredible strike.

Right-back Stephen O'Donnell tries his luck from fully 35 yards and the ball seems to take a slight deflection as it zooms past Aidan McAdams into the bottom corner.

Now it's Morton keeper Aidan McAdams performing heroics, leaping high to his right to claw a Steven Lawless header away from the top corner.

Moments earlier, Motherwell had a penalty shout turned down after Jake Hastie tumbled in the box.

Big chance for Forfar to draw level.

Substitute Scott Fenwick gets to a cross but heads over from close range.

Morton almost snatch a dramatic breakthrough.

Lewis Strapp meets a Ryan McGuffie cross with a close-range header that keeper Liam Kelly rushes out to block.

There's a distinct lack of urgency from Motherwell despite their cup hopes hanging in the balance.

Morton - second bottom of the Championship - are fewer than 10 minutes away from a penalty shootout.

Forfar are playing their fifth game in 11 days and it appears to be taking its toll now as tangerine shirts pour forward.

Lawrence Shankland has a shot blocked before Marc McNulty curls an effort round the post.

Here we go for the final 15 minutes...

Still deadlocked after the first half of extra time. Can anyone find the breakthrough?

The hosts threaten again and this time it's Nathan McGinley glancing a header narrowly past the post.

Motherwell have finally upped their game and Allan Campbell sends Jake Hastie bearing down on goal with a terrific dinked pass.

The forward fluffs his finish from the edge of the box, though, and the ball trundles wide.

Interim Inverness CT manager Neil McCann: "It's a sore one, the boys are so disappointed.

"We were confident, obviously we have to respect the Premiership side and understand it's a step up. But I'm so proud of my players because tonight you wouldn't have been able to tell the difference that we were a division down.

"It could have gone either way. We think we shaded it but big Jak Alnwick in goals has had a couple of terrific saves."

Image source, SNS

The best chance of the game by far falls to Motherwell.

Substitute Jake Hastie releases Christopher Long with a perfectly-weighted pass and the striker's shot beats the keeper but rattles off the upright.

Peter Pawlett drives Dundee United into the lead.

He collects a pass, finds a gap inside the near post, and the ball bounces off the upright on its way in.

Image source, BBC Sport

St Mirren manager Jim Goodwin is a relieved man after his side became the first through to the last eight.

"We left it late and I could stand here and talk about all the things we didn't do so well, but I'd rather congratulate Inverness on their performance," says Goodwin.

"Jak Alnwick was far too busy for my liking. We didn't underestimate them, I assure you, but they're a good side and were excellent on the night. I've got to credit my players for keeping going right to the end.

"It could have been a game that finished 5-4 but I'm delighted to win. If we go all the way no one will care about this performance."

Image source, SNS

Bernard Coll delivers a free-kick into the box after Peter Pawlett's handball, but Forfar can't make anything of it. But it's another positive start to the half from the League 1 side.

Back at it in Forfar. Is there an almighty upset brewing?

We're in for another 30 minutes at least at Fir Park as Championship strugglers Morton take top-flight Motherwell to extra time.

The visitors can be proud of their efforts, they have looked comfortable in large spells but haven't created too many clear-cut chances.

As for Motherwell, they just haven't got going at all and could be set for a shock exit if they don't improve quickly.