Forfar goalkeeper Marc McCallum takes on, and beats, two Dundee United players on a marauding run forward.

But then he passes the ball straight to Marc McNulty, and he's fortunate the United man can't get a shot away into the empty net.

Second 45 under way in Lanarkshire.

Steven Thompson
Former Scotland striker on BBC Sportsound

Forfar are continuing to make like uncomfortable for Premiership Dundee United.

A corner kick routine finds Steven Doris in space in the box, but he blasts it over the bar.

It's not a bad start for Forfar who win a free-kick in a dangerous position.

Bernard Coll curls it just over the bar.

St Mirren book a quarter-final away trip to Kilmarnock or Montrose after a dramatic late win.

Marcus Fraser tapped home in the 90th minute to complete the hosts' fightback after Nikolay Todorov had volleyed second-tier Inverness ahead.

Kristian Dennis netted a quickfire equaliser and a thrilling tie remained on a knife-edge until Fraser pounced.

Image source, sns

The visitors are not giving this up without a fight.

The ball drops to Shane Sutherland 20 yards out and his first-time strike fizzes inches wide.

Half-time looks like it might've come at the wrong time for the hosts.

They cranked up the pressure heading towards the break as Stephen O'Donnell's cross from the right found Devante Cole who couldn't steer a shot on goal.

Much improved from the Premiership side as the half-time whistle blows.

Prior to that flurry of attacking activity from the home side, you wouldn't have known who the top-flight club was if you were watching these sides for the first time.

Morton have looked incredibly comfortable and the Premiership hosts can count themselves lucky to be level.

Image source, BBC Sport

St Mirren grab a dramatic late lead - but only after rattling the woodwork yet again.

Jon Obika's deflected strike crashes off the inside of the post and substitute Brandon Mason sends the rebound back across goal where Fraser can't miss.

The third and final tie of the evening is up and running.

Goalkeeper Benjamin Siegrist has pulled out injured during the warm-up for Dundee United, meaning Deniz Mehmet has to get his gloves on just before kick-off.

Big block from the Morton defence as Motherwell start to show a bit more intent.

A corner eventually falls in the box for Nathan McGinley who takes a touch and smashes a shot towards goal, but Markus Fjortoft makes a crucial block.

St Mirren strike the goal-frame for the fourth time this evening!

Ilkay Durmus' header beats Mark Ridgers but comes back off the inside of the post and straight into the keeper's hands.

Just two minutes separated the strikes from Caley Thistle's Nikolay Todorov and St Mirren striker Kristian Dennis that have this tie on a knife edge.

Image source, sns

Image source, sns

Five changes for Forfar from their 3-1 defeat to Airdrieonians. Roberto Nditi, Bernard Coll, Jordan Allan, Scott Shepherd, Steven Doris all come into the starting line-up.

There's no place in the squad for Hamish Thomson, Andy Munro, Grant Anderson or Callum Moore. Scott Fenwick makes the bench, with just four substitutes named for the home side.

Player-manager Gary Irvine does start with his squad looking so thin.

Dundee United aren't quite as drastic, making two changes from their 1-0 win over Hamilton.

Peter Pawlett and Nicky Clark are back in the starting line-up, with Jeando Fuchs and Ian Harkes dropping to the bench.

Aidan Nesbitt is giving Stephen O'Donnell a tough evening so far.

Again the Morton winger beats his man and guides in a ball. This time he finds Robbie Muirhead who tries to scorpion kick the ball into the net but it loops over. Decent effort.

Inverness are in the ascendancy now and Scott Allan sends a low drive whistling just past the far post after some neat build-up play.

Forfar: McCallum, Nditi, Meechan, Irvine, Coll, Shepherd, MacKintosh, Holmes, Scally, Doris, Allan.

Substitutes: Hill, Hoban, Fenwick, Northcott.

Dundee United: Siegrist, L Smith, Edwards, Reynolds, J Robson, Butcher, Pawlett, Sporle, Clark, McNulty, Shankland.

Substitutes: Connolly, Mehmet, Bolton, Harkes, Appere, Watson, Hutchinson, K Smith, Fuchs.

Huge chance for the visitors!

Slack play from Robbie Crawford allows Reece Lyon to break through on goal after knocking the ball past Declan Gallagher, but Liam Kelly gets out well to smother the attempt.

Motherwell living dangerously.

After a couple of big let-offs at the other end, Inverness come out fighting and Daniel MacKay lets fly from a tight angle to force Jak Alnwick to tip over.